Add 'digest' field to PushInfo provider.

[ceason]

This PR adds a 'digest' field to PushInfo provider. The field's value is a <File> containing the to-be-published image digest.

I rearranged the container_pusher implementation function a bit, to emphasize that the digester needs to receive the same arguments as the pusher.

[k8s-ci-robot]

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[nlopezgi]

change looks good overall, but we need tests, and I just noticed we have no tests at all for container_push. I'll figure out hot to add a generic test for container_push that you can then extend. However, to test this PR, I think the easiest path is to make the image_digest an explicit output of container_push so that we can do bazel build <some container_push target> and then check that the output digest file was produced with some expected values with a file_test. I'll get back in a bit once I figure out how to best add a test for container_push, but you can get started on making the file an output.

[nlopezgi]

Also, please ignore the buildkite failures, there's an upstream issue with buildkite that is impacting all bazelbuild projects which should be fixed soon.

[nlopezgi]

created #595 to add a test

[nlopezgi]

Test for container_push has been added, let me know if you need help setting up a test to validate your PR.

[ceason]

Thanks, I've added the named output and should be able to get to the test a bit later today or some time tomorrow.

[nlopezgi]

Thanks, setting up the test should not be hard, let me know if you run into issues or have questions.

[ceason]

I've added a file_test. Thanks for the suggestion -- named output+file_test was definitely the easy path :)

[nlopezgi]

thanks for adding the test, surprisingly, it passed on BuildKite CI (I was expecting this to fail as I thought we needed to explicitly start up the local registry, but looks like it just works as it is, so I can remove the e2e test I added in an upcoming PR)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

New changes are detected. LGTM label has been removed.