gitlab_runner: Update for changed volume pruning behavior in Docker 23.0

"docker system prune --volumes" does no longer prune named volumes in
Docker 23.0[1][2], so use "docker volume prune --all"[3] for pruning
named volumes.

[1] https://github.com/docker/cli/issues/4028
[2] https://github.com/moby/moby/pull/44259
[3] https://github.com/docker/cli/pull/4229