Fixes <https://bugs.gnu.org/37161>.
* guix/scripts/pack.scm (docker-image)[build]: Add a 'directory' entry
for "/tmp" to DIRECTIVES.
* tests/pack.scm ("docker-image + localstatedir"): Test the presence of /tmp.
* gnu/tests/docker.scm (run-docker-test)["Load docker image and run
it"]: Test the presence and permission bits of "/tmp".