Make self healing capabilities more robust (#960)

In case a container image is corrupted `docker inspect` might fail:
  # docker inspect --format='{{.Id}}' "${SUPERVISOR_IMAGE}"

  Error response from daemon: readlink /mnt/data/docker/overlay2: invalid argument

In that same state the `docker images` command still shows the images.
Since `docker inspect` returns an error SUPERVISOR_IMAGE_ID will be empty
and a simple `docker pull` will be attempted. That does not suffice to
recover from a corrupted container image.

Use `docker images` to get the image ids and make sure to delete all
image ids found by that command.

Also don't use RuntimeDirectory since it deletes the runtime directory
between the service start attempts which defeats the purpose.

buildroot-external/rootfs-overlay/usr/lib/systemd/system/hassos-supervisor.service

@@ -13,7 +13,6 @@ ConditionPathExists=/run/docker.sock
 Type=simple
 Restart=always
 RestartSec=5s
-RuntimeDirectory=supervisor
 ExecStartPre=-/usr/bin/docker stop hassio_supervisor
 ExecStart=/usr/sbin/hassos-supervisor
 ExecStop=-/usr/bin/docker stop hassio_supervisor