Add git-email in dockerfile to allow git send-email from docker image

[jmurphy5]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

To enable git send-email while using the docker image, added git-email and libmailtools-perl to the Dockerfile. When the container is built it allows for sending git diffs via git send-email.

[codecov-commenter]

Codecov Report

Merging #2193 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2193   +/-   ##
=======================================
  Coverage   63.32%   63.32%           
=======================================
  Files          30       30           
  Lines        1497     1497           
=======================================
  Hits          948      948           
  Misses        549      549           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update be01808...6e72182. Read the comment docs.

[wk]

Thanks! Pulling this in since it's a small dependency in an (unfortunately) large container; please consider documenting this use case for those who prefer to use this method over the mail sending scripts in extra.