The samba-fips task would fail with a timeout connecting to samr, and
this only happened on Rackspace runners.  The GitLab shared runners were
fine, and so the main difference would be the kernel.  This gets the same
kernel as on git.samba.org, which is also helpful.

(As the image inside runs in docker, the main thing brought by the OS
docker is run in is the kernel)

We also no longer pin to an old Docker version.  This also means we do
not have access into the host to setup rng-tools, but this seems OK
for now.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>