Github actions and Github container repository

Repo to test/demo the following as Github worklfows:

• Login to ghcr.io as Docker registry
• Build and push Docker image to ghcr.io
• Clean up old versions via own action

Notes

• To use ghcr.io as Docker registry and push images, the GITHUB_TOKEN is sufficient
• To manage containers on ghcr.io via REST API, a PAT is required