FEAT: Provide Dockerimage + Chart in public registry

[uwe-mayer]

Currently to consume DISCO you need to build and host Docker image and Chart yourself.

The Chart in sapcc/helm-charts references a images hosted on keppel.

We would like to consume a public registry...

[auhlig]

i did push it now to ghcr.io/sapcc/disco and added a badge in the readme.
the workflow is broken and needs fixing