Add GitHub Action for building and pushing images

[xmudrii]

This PR adds a GitHub Action for building and pushing images. The image is built using docker buildx and pushed to GHCR as ghcr.io/kcp-dev/publishing-bot. The image is built for AMD64 and ARM64 platforms. Tags in use are:

• latest - when pushing to master
• tag name - when pushing a tag
• commit's SHA - in all cases

We're pushing the latest tag because it makes configuring and running publishing-bot easier. If we were using only SHA or tag, we would have to change the publishing-bot configuration in a separate PR after each successful push; I don't think we really need that here, we can just use latest.

The action is ran on pull requests for changes to this action and Dockerfile, on each push to master, and on semver tags. Push is done only for pushes to master and on semver tags.

Note: I'll create a follow up PR to update Makefile to use docker buildx

/assign @xrstf

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xmudrii

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[xmudrii]

Sorry, I created this PR in a wrong repo, I was supposed to create it in https://github.com/kcp-dev/publishing-bot instead!
/close

[k8s-ci-robot]

@xmudrii: Closed this PR.

In response to this:

Sorry, I created this PR in a wrong repo, I was supposed to create it in https://github.com/kcp-dev/publishing-bot instead!
/close

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