Idea: Specify base image using ko:// URL

[imjasonh]

tl;dr:

baseImageOverrides:
  github.com/my-user/my-repo/cmd/a: ko://github.com/my-user/my-repo/cmd/b

When building ./cmd/a, this would first build and publish ./cmd/b, and use that image as the base for ./cmd/a.

We'd need to detect and prevent cycles in this case, e.g.:

defaultBaseImage: ko://github.com/my-user/my-repo/cmd/b

(where baseImageOverrides doesn't specify a different base for ./cmd/b)

or:

baseImageOverrides:
  github.com/my-user/my-repo/cmd/a: ko://github.com/my-user/my-repo/cmd/b
  github.com/my-user/my-repo/cmd/b: ko://github.com/my-user/my-repo/cmd/a

This is a potential solution to #251

[github-actions]

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