This commit introduces support for configuring mount propagation when
mounting volumes with the `volume_mount` stanza on Linux targets.

Similar to Kubernetes, we expose 3 options for configuring mount
propagation:

- private, which is equivalent to `rprivate` on Linux, which does not allow the
           container to see any new nested mounts after the chroot was created.

- host-to-task, which is equivalent to `rslave` on Linux, which allows new mounts
                that have been created _outside of the container_ to be visible
                inside the container after the chroot is created.

- bidirectional, which is equivalent to `rshared` on Linux, which allows both
                 the container to see new mounts created on the host, but
                 importantly _allows the container to create mounts that are
                 visible in other containers an don the host_

private and host-to-task are safe, but bidirectional mounts can be
dangerous, a...