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Generate-DockerImageVariants

github-actions github-release powershell-gallery-release

Easily generate a repository populated with Docker image variants.

Agenda

It is very common to need to have multiple variants of a docker image. This module solves this problem by creating a simple templating framework to generate a full repository:

  • Dockerfile build contexts for variants
  • README.md containing the variants' image tags
  • Optional: Continuous integration (CI) configuration files to build and push variants' images to a docker registry

Here are some real repositories generated by using this module:

Install

Open powershell or pwsh and type:

Install-Module -Name Generate-DockerImageVariants -Repository PSGallery -Scope CurrentUser -Verbose

If prompted to trust the repository, hit Y and enter.

Usage

  1. Initialize your repository

    mkdir -p /path/to/my-project
cd /path/to/my-project

  2. Initialize the ./generate folder

    Import-Module Generate-DockerImageVariants
Generate-DockerImageVariants . -Init

    Definition and template files are generated in the ./generate folder.

    ├── generate
│   ├── definitions
│   │   ├── FILES.ps1
│   │   └── VARIANTS.ps1
│   ├── functions
│   │   └── Download-Binary.ps1
│   └── templates
│       ├── Dockerfile.ps1
│       └── README.md.ps1

  3. Edit the definitions and template files in ./generate (as needed)

  4. Generate the variants:

    Generate-DockerImageVariants .

    Build contexts of variants are generated in ./variants.

    The repository tree now looks like:

    ├── README.md
├── generate
│   ├── definitions
│   │   ├── FILES.ps1
│   │   └── VARIANTS.ps1
│   ├── functions
│   │   └── Download-Binary.ps1
│   └── templates
│       ├── Dockerfile.ps1
│       └── README.md.ps1
└── variants
    └── curl
        └── Dockerfile

Directory structure

A single folder named ./generate at the base of the repository will hold all the definition and template files:

├── generate
│   ├── definitions             # Definitions folder
│   │   ├── FILES.ps1           # An optional definition file for generation of repository files
│   │   └── VARIANTS.ps1        # Variant definition file for generation of variant build context
│   ├── functions               # Functions folder (optional). Useful for Dockerfile templating
│   │   └── Some-Function.ps1   # A function to use in your templates
│   │   └── Another-Function.ps1   # A function to use in your templates
│   └── templates               # Templates folder
│       ├── Dockerfile.ps1      # Dockerfile template (shared among variants across all distros)
│       └── README.md.ps1       # README.md template

Generation of a variant

At minimum, the ./generate/definitions/VARIANTS.ps1 definition file should contain the $VARIANTS definition like this:

$VARIANTS = @(
    @{
        tag = 'sometag'
        buildContextFiles = @{
            templates = @{
                'Dockerfile' = @{
                    common = $true
                    passes = @(
                        @{
                            variables = @{}
                        }
                    )
                }
            }
        }
    }
)

The FILES.ps1 definition file is optional, but if used, at minimum it should look like:

$FILES = @(
    # Paths are relative to the base of the project
    'README.md'
)

Upon generation, the sometag variant's build context is generated in ./variants/sometag with a file Dockerfile, as well as a file ./README.md, both relative to the base of the project.

├── README.md
└── variants
|   └── sometag
|       └── Dockerfile

See this basic example.

Using custom functions

To add a custom function, add a .ps1 file in the ./generate/functions. They will be dot-sourced and available in your templates. For instance, to add two functions Some-Function and Another-Function:

├── generate
│   ├── functions               # Functions is optional. Useful for Dockerfile templating
│   │   └── Some-Function.ps1   # A function to use in your templates
│   │   └── Another-Function.ps1   # A function to use in your templates

Then simply call them in your templates.

See this basic example with function called Download-Binary and Download-Binary2.

Generation of multiple variants

Suppose we want to generate two variants tagged curl and git:

$VARIANTS = @(
    @{
        tag = 'curl'
    }
    @{
        tag = 'git'
    }
)

# This is a optional variable that sets a common buildContextFiles for all variants
# Individual variant buildContextsFiles overrides this
$VARIANTS_SHARED = @{
    buildContextFiles = @{
        templates = @{
            'Dockerfile' = @{
                common = $true
                passes = @(
                    @{
                        variables = @{}
                    }
                )
            }
        }
    }
}

Upon generation, the curl and git variants' build contexts are generated:

└── variants
|   └── curl
|       └── Dockerfile
|   └── git
|       └── Dockerfile

See this example.

Generation of a variant through file copying

Populating a build context might not always involve processing templates. Sometimes we simply want to copy a file into the build context.

To copy a file, simply use the property copies in buildContextFiles:

$VARIANTS = @(
    @{
        tag = 'curl'
        buildContextFiles = @{
            copies = @(
                '/app'
            )
        }
    }
)

This will recursively copy all descending files/folders of the ./app folder located relative to the base of the parent repository into the to-be-generated curl variant's build directory ./variants/curl as ./variants/curl/app.

See this example.

Generation of a variant through multiple template passes

$VARIANTS = @(
    @{
        tag = 'curl'
        buildContextFiles = @{
            templates = @{
                'Dockerfile' = @{
                    common = $false
                    passes = @(
                        # The first pass will generate 'Dockerfile'
                        @{
                            variables = @{
                                'foo' = 'bar'
                            }
                        }
                        # The second pass will generate 'Dockerfile.dev'
                        @{
                            variables = @{
                                'foo2' = 'bar2'
                            }
                            generatedFileNameOverride = 'Dockerfile.dev'
                        }
                    )
                }
            }
        }
    }
)

During generation, in addition to the $VARIANT object, a $PASS_VARIABLES hashtable will be in the scope of the processed Dockerfile.ps1 template. In the first pass, the value of $PASS_VARIABLES['foo'] will be bar, and the file Dockerfile will be generated in the variant's build context. In the second pass, the value of $PASS_VARIABLES['foo2'] will be bar2, and the file Dockerfile.dev will be generated in the same build context.

See this example for using multiple passes with variables.

Generation of a single variant's build context file(s) using Component-chaining

When a variant's tag contains words delimited by -, it is known as Component-chaining. The final generated file will be a concatanation of the product of processing the template of each component specified in this chain.

For instance, suppose you want a variant Dockerfile that installs curl and git:

$VARIANTS = @(
    @{
        tag = 'curl-git'
        buildContextFiles = @{
            templates = @{
                'Dockerfile' = @{
                    common = $true
                    passes = @(
                        @{
                            variables = @{}
                        }
                    )
                }
            }
        }
    }
)

The template pass to generate the variant's build context Dockerfile proceeds as such:

  1. The template ./generate/templates/Dockerfile.ps1 is processed
  2. The build context: ./variants/curl-git is generated with the file ./variants/curl-git/Dockerfile

See this example.

Generation of multiple variants' build context file(s) using Component-chaining

$VARIANTS = @(
    @{
        tag = 'curl-git'
    }
    @{
        tag = 'curl'
    }
    @{
        tag = 'git'
    }
)

# This is a special optional variable that sets a common buildContextFiles definition for all variants
$VARIANTS_SHARED = @{
    buildContextFiles = @{
        templates = @{
            'Dockerfile' = @{
                common = $false
                passes = @(
                    @{
                        variables = @{}
                    }
                )
            }
        }
    }
}

Upon generation, three variants build contexts for variants curl-git, curl, and git are generated:

└── variants
|   └── curl-git
|       └── Dockerfile
|   └── curl
|       └── Dockerfile
|   └── git
|       └── Dockerfile

See this example.

To specify that only certain components be processed, independent of the tag property, ensure to define the components property. See these examples:

Note if using distro:

If distro is non-empty, and if the variant's tag consist of a word that matches the variant's distro, distro will not be among the components.** For instance, in the above example, if the tag is curl-git-alpine and the distro is alpine, there will still only be two components curl and git. alpine will not be considered a component.

Optional: Generate other repository files

To generate files other than variant build contexts in ./variants, define them in FILES.ps1.

Suppose we want to generate README.md:

$FILES = @(
    'README.md'
)

Then, create their templates in the ./generate/templates directory:

├── generate
│   └── templates
│       └── README.md.ps1       # README.md template

Upon generation, README.md is now generated:

└── README.md

The variables $VARIANTS will be available during the processing of the template files.

See this example.

Appendix

Variant object properties

A $VARIANT definition will contain these properties.

The buildContextFiles property of the $VARIANT object can be used to customize the template processing:

  • common - (Optional, defaults to $false) Specifies whether this file is shared by all distros. If value is $true, template has to be present in ./generate/templates/<file>.ps1. If value is $false, and if a variant distro is defined, template has to be present in ./generate/templates/<file>/<distro>/, or if a variant distro is omitted, template has to be present in ./generate/templates/<file>/<file>.ps1.
  • includeHeader - (Optional, defaults to $false) Specifies to process a template <file>.header.ps1. Template path determined by common
  • includeFooter - (Optional, defaults to $false) Specifies to process a template <file>.footer.ps1. Template path determined by common
  • passes - (Mandatory) An array of pass definitions that the template will undergo. Each pass will generate a single file.

Each template pass processes a template file <file>.ps1 template and generates a single file named <file> in the variant's build context.

A pass can be configured with the variables and generatedFileNameOverride properties.

$VARIANT = @{
    # Specifies the docker image tag
    # When the tag contains words delimited by '-', it known as component-chaining.
    tag = 'some-cool-tag'

    # Specifies a distro (optional). If you dont define a distro, templates will be sourced from ./generate/templates/<file> folder
    # In contrast, if a distro is specified, templates will be sourced from ./generate/templates/<file>/<distro> folder
    distro = 'somedistro'

    # Specifies that this variant should be tagged ':latest'. This property will be useful in generation of content in README.md or ci files. Automatically populated as $false if unspecified
    tag_as_latest = $false

    # Automatically populated
    tag_without_distro = 'somecomponent1-somecomponent2'

    # Specifies an list of components, independent of the `tag` property
    # If unspecified, this is automatically populated based on the components in the tag
    # If specified, the components override those specified in the tag
    components = @(
        'somecomponent1'
        'somecomponent2'
    )

    # Automatically populated
    build_dir_rel = './variants/distro/<tag_without_distro>'

    # Automatically populated
    build_dir = '/full/path/to/variants/distro/<tag_without_distro>'

    # Build context template definition
    buildContextFiles = @{
        templates = @{
            'Dockerfile' = @{
                # Specifies whether the template is common (shared) across distros
                common = $false

                # Specifies whether the template <file>.header.ps1 will be processed. Useful for Dockerfiles
                includeHeader = $true

                # Specifies whether the template <file>.footer.ps1 will be processed. Useful for Dockerfiles
                includeFooter = $true

                # Specifies a list of passes the template will be undergo, where each pass generates a file
                passes = @(
                    # This first pass generates the file called 'Dockerfile'
                    @{
                        # These variables will be available in $PASS_VARIABLES hashtable when this template is processed
                        variables = @{
                            foo = 'bar'
                        }

                        # If this is uncommented, the pass will generate the file named 'Dockerfile.dev' instead
                        # generatedFileNameOverride = 'Dockerfile.dev'
                    }
                )
            }
        }
        # Specifies the paths, relative to the root of the repository, to recursively copy into each variant's build context
        copies = @(
            '/app'
        )
    }
}

Debugging

Use the -Verbose switch. This gives a detail trace of:

  • Validation of $VARIANTS definition
  • Validation of $FILES definition
  • Template files or to-be-copied repository files for the build context generation
  • Template files for repository files generation

This is particularly useful when the module is throwing errors about definitions, or about missing template or to-be-copied files.

Validation of $VARIANTS and $FILES definitions

For instance, if a variant was defined with an incorrect type (expected to be hashtable):

$VARIANTS = @(
    1
    @{
        tag = 'curl'
        distro = 'alpine'
    }
)

Example of a validation trace:

PS > cd /path/to/my-repo
PS > Generate-DockerImageVariants . -Verbose
VERBOSE: Validating $VARIANTS definition
VERBOSE: Validating TargetObject '1 System.Collections.Hashtable System.Collections.Hashtable System.Collections.Hashtable System.Collections.Hashtable System.Collections.Hashtable System.Collections.Hashtable' of type 'Object[]' and basetype 'array'       against Prototype 'System.Collections.Hashtable' of type 'Object[]' and basetype 'array'
VERBOSE:        Validating TargetObject '1' of type 'Int32' and basetype 'System.ValueType'             against Prototype 'System.Collections.Hashtable' of type 'Hashtable' and basetype 'System.Object'
WARNING: Failed with errors. Exception: Type System.Int32 is invalid! It should be of type 'System.Collections.Hashtable'.

This demonstrates that a variant definition has to be of type hashtable. The value 1 is of type int32, and hence is invalid.

Validation of template files or to-be-copied files

Example of a validation trace:

PS > cd /path/to/my-repo
PS > Generate-DockerImageVariants . -Init
PS > Generate-DockerImageVariants . -Verbose
VERBOSE: Validating $VARIANTS definition
...
VERBOSE: Validating $FILES definition
...
Generating build context of variant 'curl': /path/to/my-repo/variants/curl
VERBOSE: Generating build context file: /path/to/my-repo/variants/curl/Dockerfile
VERBOSE: Processing template file: /path/to/my-repo/generate/templates/Dockerfile.ps1
Generating build context of variant 'curl-git': /path/to/my-repo/variants/curl-git
VERBOSE: Generating build context file: /path/to/my-repo/variants/curl-git/Dockerfile
VERBOSE: Processing template file: /path/to/my-repo/generate/templates/Dockerfile.ps1
Generating build context of variant 'my-cool-variant': /path/to/my-repo/variants/my-cool-variant
VERBOSE: Generating build context file: /path/to/my-repo/variants/my-cool-variant/Dockerfile
VERBOSE: Processing template file: /path/to/my-repo/generate/templates/Dockerfile.ps1
Generating repository file: /path/to/my-repo/README.md
VERBOSE: Processing template file: /path/to/my-repo/generate/templates/README.md.ps1