Half way transition to docker compose v2 for volume names

With the last MR !143 (merged) I ensured that the docker volume for the static backend files was removed.

However, after this deployment the name of that volume changed (due to the docker compose v2 that we now use).

For staging & staging-dev I can now switch completely to the new name. For prod I think it is more robust to check for both possible volumes names (at least until we made the next deployment).