CI: Build docker images in GHA, store cache inline and push to GHCR

[effigies]

This PR is a potential model for nipreps using GHA to build Docker images. The latest build (from a previous commit's cache) took 2 minutes, including checkout, build and push. If the cache needs rebuilding, it's <10 minutes. This is a vast improvement from Circle all on it's own.

I will make a separate PR to start triggering Circle builds from GHA after build. I would like to get to a model of:

graph LR;
  subgraph GitHub;
    test & build
  end
  subgraph Circle
    build --> ds005 & ds054 & ds210
  end
  subgraph gh2["GitHub"]
    test & ds005 & ds054 & ds210 --> deploy
  end

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Where Circle runs anything where we want inspectable artifacts and nothing else. For SDCflows, it would be tests again, but with artifact saving turned on.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.06%. Comparing base (52eee57) to head (79ee1b6).
Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3380   +/-   ##
=======================================
  Coverage   72.06%   72.06%           
=======================================
  Files          57       57           
  Lines        4231     4231           
  Branches      456      456           
=======================================
  Hits         3049     3049           
  Misses       1067     1067           
  Partials      115      115           

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