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.