Add option to skip image build e.g skip-build: true/false

[denismatiu]

We build the image step earlier and don't want to rebuild the image but only pass its name and tag

[johanneswuerbach]

While working on this in #40 we had one question, do you still want your image to be pushed to the registry or would you expect the action to take care of this?

You can also try the WIP branch with humanitec/build-push-to-humanitec@existing-image, just keep in mind that it will be deleted once merged.

[denismatiu]

@johanneswuerbach No, just passing the name and image tag. Build and pushing the image is done in another step in our case

[johanneswuerbach]

You can now provide an existing image using existing-image. In case the image is already pushed to the registry, the provided version and the :version tag must be matching.

E.g.

uses: humanitec/build-push-to-humanitec@v1
with:
  existing-image: europe-west3-docker.pkg.dev/gcp-project/repository/repo:5
  external-registry-url: europe-west3-docker.pkg.dev/gcp-project/repository
  version: 5