Allow 'instance' parameter in database.yml to be set via environment variable

[buhlmann-thomas-rix]

When using MS SQL Server with a named instance as database server, it is required to set the name of the instance in the instance parameter in config/database.yml. Using sql-server-host\instance_name for host parameter does not work.

Please introduce a new environment variable, e.g. REDMINE_DB_SQLSERVER_INSTANCE, that allows setting the name of the database instance to connect to.

Currently, I created a workaround by exporting the config/database.yml file via a Docker volume to my local filesystem and altering its contents manually. But this should not be the official way doing this.

Working example:

production:
  adapter: "sqlserver"
  host: "sql-server-host"
  instance: "instance_name"
  port: "61602"
  username: "sql-username"
  password: "super-secret-sql-password"
  database: "REDMINE"
  encoding: "utf-8"

[LaurentGoderre]

I believe it's reasonable to rely on the config file for db connection that require more complex config.

[buhlmann-thomas-rix]

Well, having a named instance SQL Server is not really a rare case.

IMHO, the way to configure the DB connection should be the same for all setups and not "This is your standard case, but if your setup differs from this, you have to do something completely different (and this is not even documented anywhere)".