feat: remote build private repo support

[lkingland]

• 🎁 allow remote builds to access private repositories

By setting the DOCKER_CONFIG prior to the "buildah bud" command being, it should pick up authentication necessary for accessing private repositories.

/kind enhancement

[knative-prow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.40%. Comparing base (bf9a70a) to head (556927f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2580      +/-   ##
==========================================
+ Coverage   64.12%   65.40%   +1.28%     
==========================================
  Files         130      130              
  Lines       15485    15494       +9     
==========================================
+ Hits         9929    10134     +205     
+ Misses       4619     4385     -234     
- Partials      937      975      +38     

Flag	Coverage Δ
e2e-test	35.87% <0.00%> (-0.03%)	⬇️
e2e-test-oncluster	32.89% <100.00%> (+0.07%)	⬆️
e2e-test-oncluster-runtime	28.68% <100.00%> (?)
e2e-test-runtime-go	26.43% <0.00%> (?)
e2e-test-runtime-node	25.83% <0.00%> (?)
e2e-test-runtime-python	25.83% <0.00%> (?)
e2e-test-runtime-quarkus	25.98% <0.00%> (?)
e2e-test-runtime-rust	24.97% <0.00%> (?)
e2e-test-runtime-springboot	25.01% <0.00%> (?)
e2e-test-runtime-typescript	25.94% <0.00%> (?)
integration-tests	51.96% <100.00%> (+2.31%)	⬆️
unit-tests	50.89% <100.00%> (?)
unit-tests-macos-latest	?
unit-tests-ubuntu-latest	?
unit-tests-windows-latest	?

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[matejvasek]

/lgtm