docker: add ownership regression test

[tormath1]

This prevent regression like the one reported in flatcar/Flatcar#1203 to occur again.

Testing done

It fails, as expected, on Stable with:

$ kola run --qemu-image ./stable/flatcar_production_qemu_image.img docker.base
=== RUN   docker.base
=== RUN   docker.base/ownership
--- FAIL: docker.base (38.08s)
...
            docker.go:721: ownership should be conserved between run and export of Docker images. Got: 0/42 and 0/0
FAIL, output in _kola_temp/qemu-2023-10-09-1956-136705

Pass on Alpha (and certainly Beta):

$ kola run --qemu-image ./alpha/flatcar_production_qemu_image.img docker.base
=== RUN   docker.base
=== RUN   docker.base/ownership
--- PASS: docker.base (29.75s)
    --- PASS: docker.base/ownership (10.82s)
...
PASS, output in _kola_temp/qemu-2023-10-09-1956-136583

[pothos]

I'm not a large fan of regression tests for mainly upstream issues, specially when they are so specific that they probably won't happen exactly this way again but different. Anyway, if this function won't take long to execute it's ok to have it.

[tormath1]

I get your point. By doing this we can prevent this regression again (even if there is a small probability that it occurs again as you said) but we can also catch other issues caused by this scenario.
The test only runs on qemu as part of the docker base tests, the only "long" action is the pulling of the nginx image, the rest is super fast.

[t-lo]

LGTM, thank you @tormath1!