Handle applications that have already vendored their bundle or set a local bundle config

[dazuma]

Received a customer report that a deployed application could not load its bundled gems. Bundler appeared to succeed in installing the gems, but the application could not find them during Bundler.require. This is not certain, but it is possible this was because vestiges of an earlier local vendored bundle (e.g. vendor/bundle and/or .bundle directories) were present in the application directory prior to deployment.

We should investigate what happens in such a case, and make sure the runtime handles it. In particular:

• If there is a vendored bundle installation already
• If such an installation is in a nonstandard location (and thus might not get copied over with the app during the docker build)
• If there are important bundler settings in a local bundler config that need to be preserved