This PR moves our algorithm image build infrastructure from our internal Gitlab instance to GitHub Actions (#25).

Additions:

• use GitHub actions to build and test PR algorithm images
• use GitHub actions to publish algorithm images to GitHub packages once the PR is merged to main
• adapt and publish base images in this PR, others will follow

Changes to internal build process:

• We don't publish temporary images to the registry.
• We add helpful labels to the images (based on OCI standard).
• Each image is built and versioned independently. We use the version-key in the manifest of algorithms and a separate version.txt-file for our base images.

Some limitation:
Jobs always require the correct version of base images already available at the registry. This means if a PR introduces changes to, e.g., lof, the requested version of the ghcr.io/timeeval/pyod base image must be available from the registry (pull must succeed). This means that changes to images must be made in consecutive PRs:

1. base images
2. intermediate images
3. algorithm images

I will use this order for the introduction of the CI changes as well.