[ci] replace Docker with ko for image build and push

[moshevayner]

Signed-off-by: Moshe Vayner moshe@vayner.me

General:

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
4. Are your commits atomic, addressing one change per commit?
5. Are you following the conventions of the language?
6. Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. Have you explained your rationale for why this feature is needed?
8. Have you linked your PR to an open issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.65%. Comparing base (68eacfd) to head (a36e155).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #582   +/-   ##
=======================================
  Coverage   73.65%   73.65%           
=======================================
  Files          19       19           
  Lines        3029     3029           
=======================================
  Hits         2231     2231           
  Misses        538      538           
  Partials      260      260           

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