Justification for building a custom Docker image instead of using the canonical Ruby one

[seeflanigan]

Per docs for the canonical Ruby image. "The onbuild tag expects a Gemfile.lock in your app directory."

Because Gemfile.lock is presently not versioned, building an image for suture based on the Ruby image results in the following fatal error:

Status: Downloaded newer image for ruby:2.1-onbuild
# Executing 4 build triggers...
Step 1 : COPY Gemfile /usr/src/app/
Step 1 : COPY Gemfile.lock /usr/src/app/
lstat Gemfile.lock: no such file or directory

There are workarounds, building a custom image instead of the conventional one. However, by following the idiomatic approach of using the official Ruby image, we defer some liability for incidental complexity to the official image maintainers.

However according to this it is recommended not to version Gemfile.lock for library development.

Therefore, I will leave this note as a reminder to myself and future maintainers as to the justification for building a custom image in favor of using the onbuild tag.