docker: don't install copr-selinux into containers
SELinux is _not_ running inside a consider, but on host (outside). Not
only the installation of copr-selinux is useless inside -- there also
are some weird problems with the installation of selinux-policy package
(transitive dep) inside a buildah container.
We already use conditional Requires: in Copr packages, but there's still
a problem in dist-git.spec: dist-git brings -> dist-git-selinux ->
selinux-policy. This causes container image build problems. Will be
fixed here:
https://github.com/release-engineering/dist-git/pull/40