Add support for SELinux systems

[YtvwlD]

This is the other half of the changes we talked about last week: Since we're running the new VM with RHEL, we need some adjustments for SELinux. These are ignored on systems without it.

Adding Z allows this container (and only this one) to access the bind mount. This seems to be only needed for files in /srv, because RHEL ships a special label for this folder.
This is also why /srv/docker/bridgehead/bridgehead can be run manually, but not from a systemd service.

We additionally had the slight problem that with Podman, /var/run/docker.sock, being a symlink to /var/run/podman/podman.sock, was owned by root:root. Changing this to root:docker worked, but I'm not quite sure where to put it in the existing scripts. (docker is the usual Docker cli.)

@Threated

[Threated]

Thanks a lot for the PR ❤️
Which volumes need the Z flag I saw you only added it to volumes that point to /srv/docker/bridghead/* so I guess /etc/bridghead/* is fine?
Also in other projects we have mounts in /tmp/bridgehead/* and /var/cache/bridgehead/* (I also changed the dnpm docker volumes to be fs mounts to that dir today as it was requested by Martin in the other PR) I assume that is problematic as well?
I wonder if we can get away with just executing a few commands as root when running sudo ./bridgehead install <project> to fix all the SEL specialties without needing to remember to add the z flag to each volume or not being able to use docker secrets 🤔

[YtvwlD]

(This is a rebase to keep the diff readable.)

[YtvwlD]

After taking a look at the contexts of the files, I'm not sure that we can solve this by just changing the files.
After cloning, everything is unconfined_u:object_r:var_t:s0.
But when the containers are running (with :Z), the files get different contexts:
ccp/root.crt.pem gets system_u:object_r:container_file_t:s0:c252,c820
minimal/modules/dnpm-central-targets.json gets system_u:object_r:container_file_t:s0:c246,c734
and these numbers change on container restart!

Someone with knowledge of selinux might be able to find a common label that makes the files available to all containers, but I can't. :) And I don't think a Z really hurts.

The Z is not required for files in /etc, I've removed it for proxy.pem.
I'm not sure why using Docker Secrets doesn't work here. Is this more than a normal mount?

I still need to take a look at /var/cache.

[YtvwlD]

We seem to need the Z for /var/cache as well. The containers are not able to write to the folders otherwise.

[TKussel]

@YtvwlD could you please rebase your branch based on the new feat/selinux branch? We are increasingly running into SELinux issues in other projects and would like to use your PR as a basis to extend it to all other projects as well.

[TKussel]

As @Threated tells me, this is actually a horror rebase. I will try to cherry pick your commits into the branch. As this PR has relevant discussion, I leave it open for now.