Much like ~/.gemrc, it is useful to be able to specify additional sources to be used in a particular environment. This would be helpful to solve the issue of wanting to keep private sources out of a Gemfile.
One solution is discussed here: http://mojinations.com/blog/2012/07/09/keeping-private-repo-passwords-out-of-your-gemfile/
However, this promotes the private repository to Gemfile.lock. I originally approached this by trying to declare some repositories in Gemfile as private, but this overly complicates the comparison of the Gemfile and Gemfile.lock for differences.
Instead, I propose promoting a similar option from the environment as a way of adding repositories that are not declared in either file:
bundle config env_sources source1,source2,...
These should be used to resolve gems, but not added to the lock file, on the assumption that everyone building the project must add the sources with their own credentials.
There may be a better name for the argument 😃
Discussion moved to #2141.