Docker: Use one process to decrease memory usage.

[marcfehling]

This is another attempt to get the docker workflow fixed. The message we get from github is rather vague:

The hosted runner: GitHub Actions X lost communication with the server. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.

I believe the only thing that could potentially fail from the description is that we run out of memory. I suggest to limit the number of processes explicitly to 1 with which we build deal.II in the Dockerfile.

@masterleinad can build the Docker image without problems, see #16047 (comment). It could be the system specs of the github machines that cause this.

Probably a nicer approach is to introduce a new argument for the number of processes to remain flexible, i.e., ARG NPROCS=2, and then set it explicitly to 1 for the focal image. But we could leave that for a follow-up PR. Let's try this out first!

[masterleinad]

Let's try it!

[marcfehling]

Reducing the number of jobs helped to prevent the focal build from crashing. There was an uploading issue for the jammy build, but I believe it's just a temporary problem. I call this a success!
https://github.com/dealii/dealii/actions/runs/6342478809

Runtime increased substantially as a result from using only one job. I will prepare some follow-up PRs to mitigate the impact.