Previously only `/var` was `rshared` but some people need to share
mounts in `/opt` etc so let us make everything `rshared` for now.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
Instead of mounting a new filesystem, revert to doing a `rw` bind.
However do not make `/` `rshared`, just `/var` as that is where we expect
filesystems to be mounted for persistence. Also only make the actual
container rootfs writeable, not the whole directory.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
