Add Linux As A Bundler Platform

With a locally developed Ruby project on Mac OSX, I have darwin specified as the platform in the Gemfile.lock.

PLATFORMS
  x86_64-darwin-19

When setting up CI for my project on a linux container, I'd get an error like this:

Your bundle only supports platforms ["x86_64-darwin-19"] ...

This platform incompatability can be solved with by adding linux as a platform and re-resolving the lock file. This is done with bundle lock and the --add-platform flag.

$ bundle lock --add-platform x86_64-linux

If all existing gems work with the new linux platform, the command will succeed and the updated Gemfile.lock will have the following PLATFORMS section.

PLATFORMS
  x86_64-darwin-19
  x86_64-linux

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