Automatically push docker images during release CI/CD

[VJftw]

This PR adds the ./pleasew run //scripts:publish-images to the release workflow so that Docker images are automatically pushed via CI/CD for each release. I have added the following secrets to the repository's actions for this to work:

• DOCKERHUB_USERNAME: vjftw (my Docker Hub account).
• DOCKERHUB_PASSWORD: A unique secret access token only used for this repository's GitHub Actions. (https://docs.docker.com/docker-hub/access-tokens/)

From the "Secrets" page in "Settings" for this repository:

Secrets are environment variables that are encrypted and only exposed to selected actions. Anyone with collaborator access to this repository can use these secrets in a workflow.
Secrets are not passed to workflows that are triggered by a pull request from a fork.

[northdpole]

good stuff, we needed this :D

[VJftw]

sorry, thought it would be best to test the entire cycle on my https://github.com/VJftw/docker-registry-proxy project first and discovered that we need to run git fetch --tags --force on GitHub Actions workers as a workaround for not having all the tags (see actions/checkout#290). This works as expected now. I think in the future, we could remove the on.push.branches: [master] if the releases get a bit too spammy - but we can decide in the future