Dockerfile: limit make parallelism

[pjonsson]

Build time improves if the number
of compilation procesess aren't
unlimited, so put an upper limit
of the number of cores on the build
machine.

[pjonsson]

@davidfrantz this PR is unrelated to the image upgrade, so it should be possible to merge.

If your CI actions run this code, you should see a reasonable performance improvement of not starting 20+ compilers at the same time since the Github CI runners only have 2-3 cores.

[davidfrantz]

thanks @pjonsson.

I think I have misunderstood make's -j option. I thought it would do exactly that