Use latest ubuntu image in docker.yml.

[marcfehling]

OS of the host system is not important.

Refers to #15060 (comment) and #15074 (comment).

[peterrum]

We can try this. But different version of ubuntu might have different modules shipped.

[marcfehling]

This PR reverts a change in #15060 and puts ubuntu-latest in place as before.

As @tjhei pointed out in #15074 (comment), the OS of the host system is not important when building the docker image. The yaml script enhances a pre-existing image with all necessary modules, i.e. dealii/dependencies:focal. So the proper OS and corresponding modules are already there, the deal.II installation will be added and then the image will be uploaded to dockerhub. (Please correct me if my conception of the whole docker workflow is still wrong.)

[marcfehling]

Since #15060, the github-docker workers got aborted as no workers were available.

This request was automatically failed because there were no enabled runners online to process the request for more than 1 days.

Other workers pass, so this leaves me puzzled. This is just a shot in the dark, but maybe the change to ubuntu-20.04 is responsible for that. One more reason to merge this PR quickly...