A local Gemfile where you can add gems that you don't want to add to the repository
Copy Gemfile.local into project root and add the following to your .gitignore:
/Gemfile.local
/Gemfile.local.lock
Also, you might want to duplicate your project's Gemfile.lock into Gemfile.local.lock to make sure gems are locked under the same versions. In your terminal run:
cp Gemfile.lock Gemfile.local.lock
To use Gemfile.local run:
bundle --gemfile Gemfile.local
Note: This should store a config in the .bundle/config file and then read from that, however I couldn't get this to work for the BUNDLE_GEMFILE config no matter what I tried. There is a bug request to add this as standard functionality to Bundler, however it appears there is nobody around to work on it: rubygems/bundler#183 (comment)