This buildback is always enabled, i.e. when it's added to an app, the app will always be detected by Heroku as a "multi Gemfile" app, even if it's not.

It will however have no effect if the BUNDLE_GEMFILE environment variable is not defined.

Ideally, we would only enable it when BUNDLE_GEMFILE is defined, but environment variables are not available in bin/detect (see this issue).

If BUNDLE_GEMFILE is present, it makes creates the following symlinks :
Gemfile → $BUNDLE_GEMFILE
Gemfile.lock → $BUNDLE_GEMILE.lock

Note: the symlinks are only created if the source and the target are not already linked together (in any direction).