Fix docker push github action and expand with additional metadata

[ebr]

Modifies the cloud image build workflow; rules are as follows:

• image will be built on all pushes to main, all tags starting with v, and all pull requests.
• image will NOT be pushed on any pull requests.
• do NOT perform docker login on pull requests.
• PR workflows from new contributors will need approval to run. Workflows from forked repo will run in the context of the fork first, and will not have access to any secrets in this repo. Collaborators can examine them before approving workflow runs on this repo.

Image tags added as follows:

• :latest for the last commit on the repo (any branch or tag, even though it's just main now)
• :edge for every last commit on main (like :latest but only for main)
• :main (or other <branch_name>, (even thouh we only run this on main right now)
• :v1.2.3
• 1.2.3
• 1.2
• cdf4567 (git short SHA)

(I'm open to disabling workflow execution on PRs if that is deemed uncomfortable)