Wrong tag name for z3 docker images on github

[gtrepta]

The github action for building the Docker images is tagging the z3 image with the ubuntu distro name, but that tag doesn't get used anywhere else, neither here nor in other projects. I think this hasn't caused builds to fail because other projects have built the cached docker image with the proper tag.

Here's a snippet from a recent build log showcasing the issue:

docker build . ... --tag z3:jammy-4.8.15 --file .github/workflows/Dockerfile.z3

...

docker build . --file .github/workflows/Dockerfile ...
...
#4 [internal] load metadata for docker.io/library/z3:4.8.15

...