Had to bind /var/run/docker.sock inside CentOS container so
it can use the host's docker daemon for testing, see
https://jpetazzo.github.io/2015/09/03/do-not-use-docker-in-docker-for-ci/
for rationale.
Unlike that article, mapping directly to /var/run/docker.sock didn't work
here, cause running CentOS' own /usr/sbin/init mounts another overlayfs
which shadows /var/run/.
Instead, I bound it to /docker.sock, set DOCKER_HOST=unix:///docker.sock
and cleared the path in several places for it to be exported all the way
to the relevant tests.

Now, this would've been a much easier change if the test container started
the build script directly, instead of running /usr/sbin/init, but since
we're trying simulate a full operating system running, doing that would've
reduced the value of the test.