Make docker CLI tests faster by mocking the call to docker auth

[tejal29]

Before, on master

ok  	github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/docker	46.868s
=== Slow Tests ===
37.05s	TestDockerCLIBuild
8.05s	TestDockerCLIBuild/docker_build
8.04s	TestDockerBuildError
8.03s	TestDockerBuildError/docker_file_present
7.67s	TestDockerCLIBuild/extra_env
7.55s	TestDockerCLIBuild/buildkit
6.91s	TestDockerCLIBuild/buildkit_and_extra_env
6.87s	TestDockerCLIBuild/env_var_collisions
4.44s	TestGetAllAuthConfigs

After this change,

ok  	github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/docker	1.453s

/cc @marlon-gamez

[codecov]

Codecov Report

Merging #5060 (9c63f38) into master (3fe0c99) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5060      +/-   ##
==========================================
+ Coverage   72.27%   72.29%   +0.02%     
==========================================
  Files         368      368              
  Lines       12914    12914              
==========================================
+ Hits         9333     9336       +3     
+ Misses       2890     2889       -1     
+ Partials      691      689       -2     

Impacted Files	Coverage Δ
pkg/skaffold/docker/image.go	80.93% <0.00%> (+1.39%)	⬆️

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[MarlonGamez]

nice! Thanks for this :) I'll approve once CI passes