[CI][Testing] Add BuildJet CI runners for unit tests, integration tests; cache Docker images - backport

[peterargue]

Backports: #4906

[peterargue]

The Epoch Cohort2 test keeps failing, so I'm guessing there are some changes missing. @jordanschalm are you aware of any change on master that may affect them? backporting this will make the v0.32 tests less flaky

[jordanschalm]

Probably this one: #4975

[jordanschalm]

@peterargue I cherry-picked #4975 into this branch, in this commit: 9c4f2c3. That should resolve the flaky test 🤞

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4805471) 55.84% compared to head (9c4f2c3) 57.23%.
Report is 19 commits behind head on v0.32.

Files	Patch %	Lines
utils/test_matrix/test_matrix.go	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            v0.32    #5015      +/-   ##
==========================================
+ Coverage   55.84%   57.23%   +1.38%     
==========================================
  Files         945      759     -186     
  Lines       88208    71988   -16220     
==========================================
- Hits        49257    41199    -8058     
+ Misses      35241    27660    -7581     
+ Partials     3710     3129     -581     

Flag	Coverage Δ
unittests	57.23% <96.66%> (+1.38%)	⬆️

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[peterargue]

@peterargue I cherry-picked #4975 into this branch, in this commit: 9c4f2c3. That should resolve the flaky test 🤞

thanks!