Enable customizations to .gitignore

[dmikusa]

Describe the Enhancement

Right now, pipeline-builder takes over total control over the .gitignore file. This is not necessarily a problem, but some buildpacks have additions to the .gitignore file.

We should add a way that allows individual buildpacks to add to the .gitignore file.

Possible Solution

Turn the .gitignore file supplied by pipeline-builder into a Go template file. The pipeline-descriptor.yml can then be modified to include an optional section where the use can specify before and after lines which would go before and after the standard lines.

Motivation

Support some basic level of customization for individual buildpacks.