feat: unionize Ubuntu Containerfile with the one for Alpine

[rwaffen]

Short description

unionize Ubuntu Containfile with the one for Alpine. One step more to rootless containers. One step more to only one container file again.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rwaffen]

I would like to have this PR a bit longer open, so that people might discuss or have opinions... like in #124