Build on Eric's efforts.

[UserNotFound]

Building on Eric's efforts:

• Added the Gemfile.lock, otherwise the Compose volume mount overwrites (effectively deletes) the lock created during build (which would have the effect of forcing you to bundle install again the first time you use the container)
• Use a simpler method for reverting to an older Bundler version (1.17.3)
• install dependencies before copying code, to avoid rebuilding the bundle install layer every time you edit code
• Mount your current tokens, rather than requiring you to set it via ENV
• Add a Makefile with common actions
• Set a CMD that works (though still not useful unless you mount a volume or pass creds via ENV)
• added a Dockerignore to keep the image tidy.

[neurosnap]

Resolved conflicts in #333