set default ruby version for image builds in Dockerfile

[alexskr]

set default ruby version for image builds in Dockerfile
follow up to PR #84

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.92%. Comparing base (64621dd) to head (9d1b6ba).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #85   +/-   ##
=======================================
  Coverage   54.92%   54.92%           
=======================================
  Files          16       16           
  Lines        1229     1229           
=======================================
  Hits          675      675           
  Misses        554      554           

Flag	Coverage Δ
unittests	54.92% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.