Refactor to multi-stage build and latest ruby container

[TheBay0r]

Hi @parabuzzle I was experimenting around with multi stage builds in docker and I'm quite satisfied with the results of it. When using the new Dockerfile the image uncompressed image size of craneoperator drops from 388MB down to 124MB

I'm not a really sure why I need the bundle install in the "running"-container. If I don't put it there, in complains that some gems need to be installed first although they are all available in the vendor/ folder. Maybe you can give me some input on that, since I'm not familiar with ruby/gem!?

Other than that everythings seems to be working fine on my machine 🙂

[parabuzzle]

This is awesome! it was on my bucket list to move my stuff to multi-stage builds.. I'll see what's going on with the gem install.. bundler can be finicky.

[TheBay0r]

@parabuzzle any news about this?

[scones]

since i am already stalking this repository: multistage builds are a good idea. with ruby 2.6 this might be just unnecessary complexity, as bundler is part of ruby with 2.6