Setup and implement publishing Docker images to DockerHub

[paullatzelsperger]

Description

When publishing Docker images to GitHub packages in an Eclipse-managed Github repository, they are automatically set to private. This is an org-wide setting and cannot be changed.

Thus, we must publish our docker images to DockerHub.

Requirements

• image namespace and name must be defined as env variables
• we must use org-provided credentials to log into Dockerhub:

   - name: DockerHub login
        if: github.event_name != 'pull_request'
        uses: docker/login-action@v2
        with:
          username: ${{ secrets.DOCKER_HUB_USER }}
          password: ${{ secrets.DOCKER_HUB_TOKEN }}

• the README.md file must contain a Notice section, similar to the IRS example
• the description (by default: README.md) must get published to DockerHub as well using a GH Actions:

    - name: Update Docker Hub description
        if: github.event_name != 'pull_request'
        uses: peter-evans/dockerhub-description@v3
        with:
          username: ${{ secrets.DOCKER_HUB_USER }}
          password: ${{ secrets.DOCKER_HUB_TOKEN }}
          repository: ${{ env.IMAGE_NAMESPACE }}/${{ env.IMAGE_NAME }}

Implementation proposal

• leave publishing to GH Packages in for now, just to be safe
• Extract building and pushing docker images to a separate GH workflow publish-docker.yaml
• publish-docker.yaml runs only on pushes to develop and main and only if the Dockerhub credentials are present (prevents runs on forks)