Add multi-arch image via Bazel

[mattmoor]

Description

This starts to add multi-arch container images built via Bazel, turned into "manifest lists" in the Docker daemon, and published.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

The executor:bazel image now supports both amd64 and arm64.

[mattmoor]

I'm staging this to test the Travis integration, but I need to make the cloudbuild.yaml changes non-invasive (right now they are breaking, and should not be merged)

cc @tejal29

[mattmoor]

Travis seems to have not gotten notified, or is otherwise upset with me... 🤔

[mattmoor]

Alright, commenting the travis changes seems to have unwedged it. I "love" silent failures 🙃

[tejal29]

Looks like we need to switch to paid tier for travis.

[mattmoor]

Alright, my stripped down Travis build passed, so I uncommented everything here: https://travis-ci.org/github/GoogleContainerTools/kaniko/builds/734073723

This should be RFAL, please look carefully at the deploy/cloudbuild_release.yaml as I did not test that due to hard-coded project and use of tags.

[mattmoor]

Everything is green! 🤩

[tejal29]

LGTM except some nits on code organization.

[mattmoor]

Alright, looks like the previous failure was a flake, so this should be RFAL.

[mattmoor]

@tejal29 LMK if these needs anything else!

[tejal29]

I will share the results of GCB tirgger with you.