Use GitHub action caching to speed up test image build

[omus]

Makes use of GitHub action caching to allow Docker image building to use layer caching.

[omus]

Code is currently not resulting in any layer cache reuse. Something isn't working here

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.86%. Comparing base (426c2bd) to head (996d6c2).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #97       +/-   ##
===========================================
+ Coverage   49.10%   66.86%   +17.75%     
===========================================
  Files           4        4               
  Lines         167      172        +5     
===========================================
+ Hits           82      115       +33     
+ Misses         85       57       -28     

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