Docker base image updates

[BWibo]

Update Docker base images to latest Java LTS v21 and latest Ubuntu.

@clausnagel
Do you recall if there was a specific reason why we choose jdk for the runtime image? Is the JDK a requirement for the tool at runtime? If no, we could save ~2/3 of the image size.

I tested to run the tool (no import or export operation), there were no errors.

[clausnagel]

Well, no, I can't remember. Let me also do some tests and post updates here.

[clausnagel]

Hi @BWibo, I cannot build a Docker image using the adapted Dockerfile. See the errors in the screenshot below. Can you reproduce these errors on your test machine? All works fine in main branch though.

[BWibo]

Oh yes, I didn't expect that and didn't test it.
The 21-jre-noble baseimage already has a user ubuntu with uid, gid 1000 resulting in a conflict. This is actually a good thing, as it saves us the creation of a user. I don't see any issues, if the tools runs as ubuntu and not citydb-tool user, as only uid and gid are relevant. I simplyified the image, it should build now.