Dockerfile/service: Add test-pack-entrypoint

test-pack-entrypoint works around some issues with running test-packs in
docker.
Dockerfile: Don't VOLUME `/var/lib/stbt/results`

`/var/lib/stbt/results` will be a volume, but shouldn't belong to the
test-pack, it should belong to its own, more longer lived container and
be mounted into the test pack by `-v host:container` or with
`--volumes-from`.
service: Require passing `stbt-ssh-endpoint` as ssh command

So it works the same whether `authorized_keys`
`command=stbt-ssh-endpoint` is used or not.