fix: skaffold's assumption for image tag when building via buildkit and custom output

[elnoro]

Fixes: #7024

Description
The original issue is about using local docker buildkit build. When building with buildkit and custom output, image is not tagged by default. Empty image id is passed to ImageAPIClient.ImageTag from the docker package, which fails.

This line suggests that an empty image id is a valid use case in some circumstances.

I simply added a new condition to the method - if image id is empty, docker ImageTag is not called and simply returns empty string instead.
Unit test included.

User facing changes (remove if N/A)
Before
When using buildkit and custom output, skaffold returned the following error
Error parsing reference: "" is not a valid repository/tag: invalid reference format
After
When using buildkit and custom output, skaffold successfully completes build.

Custom output is produced in both cases, as it's exported by the docker cli, not by skaffold

[tejal29]

Thanks @elnoro for your PR.

will try the steps mentioned in the original issue to verify.

Can you please fix the commit message to be in accordance conventionalcommits.org by using git commit --amend

[codecov]

Codecov Report

Merging #7120 (e713c4e) into main (290280e) will decrease coverage by 1.81%.
The diff coverage is 56.96%.

@@            Coverage Diff             @@
##             main    #7120      +/-   ##
==========================================
- Coverage   70.48%   68.67%   -1.82%     
==========================================
  Files         515      558      +43     
  Lines       23150    26086    +2936     
==========================================
+ Hits        16317    17914    +1597     
- Misses       5776     6941    +1165     
- Partials     1057     1231     +174     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/deploy.go	52.00% <ø> (-1.85%)	⬇️
cmd/skaffold/app/cmd/dev.go	84.61% <0.00%> (ø)
cmd/skaffold/app/cmd/render.go	36.66% <0.00%> (-4.72%)	⬇️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/fix.go	68.85% <40.00%> (-7.62%)	⬇️
cmd/skaffold/app/cmd/lint.go	42.85% <42.85%> (ø)
cmd/skaffold/app/cmd/find_configs.go	48.88% <50.00%> (+0.24%)	⬆️
cmd/skaffold/app/skaffold.go	76.19% <70.00%> (-8.43%)	⬇️
... and 209 more

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[elnoro]

Hello, commit message has been updated.

I see there is a failing unit test on windows - deploy/kustomize/kustomize_test.go running for more than 1m.
This seems unrelated to the changes, but I'll see if I can find the connection.

[tejal29]

re-running the test suite.