run-tests.yml: implement docker layer caching

[hemberger]

This GitHub Action caches all layers that are created after the step
where it is specified (which is why we pre-fetch some pull-only
images that we know we'll be using to run the tests).

The goal is faster CI by speeding up the build stage.

[codecov]

Codecov Report

Merging #996 (20da9f4) into master (2ad5d65) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #996   +/-   ##
========================================
  Coverage      2.76%   2.76%           
  Complexity     4062    4062           
========================================
  Files            74      74           
  Lines         11201   11201           
========================================
  Hits            310     310           
  Misses        10891   10891           

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[hemberger]

This seems to be reducing the build-and-test time by about 40% (~90 seconds). While this is pretty good, it's perhaps not quite enough to warrant all the extra complexity. Furthermore, there are concerns about performance degradation over time as the cache fills up with obsolete build layers (see satackey/action-docker-layer-caching#55).

For now, we'll stick with the simple, non-cached solution. But it's good to know that this works as expected should we have a more urgent need to reduce build times in the future. (It may also allow time for GitHub to provide a more "official" workflow for caching docker layers.)

First build, when the cache is empty:

Subsequent builds utilizing the cache:

[hemberger]

There are more modern solutions to docker caching here: https://github.com/docker/build-push-action/tree/master/docs/advanced

[hemberger]

Superseded by #1451.