scripts/container.sh: fix rootless Podman on systems with SELinux (#3213)

We need to pass the `z` option with the volume mount so it gets labeled
correctly and access is possible on SELinux-enabled systems. On systems
without SELinux, the flag is a no-op.

Fixes #3199

(cherry picked from commit 9ccd353e469da2dd8d57b7ff839e8fe308d3dfe1)

Co-authored-by: Matthias Schiffer <mschiffer@universe-factory.net>