Reference STOPSIGNAL by number instead of name

[lion7]

Using SIGRTMIN+3 doesn't work on k3s (and probably other runc-based runtimes). This issue is described in https://github.com/opencontainers/runc/issues/1766. The suggested workaround there is to reference the signal by number instead: STOPSIGNAL 37. https://github.com/alehaa/docker-debian-systemd/blob/bd46fcb53aad236ea17efeddc26e9ae2242f017e/Dockerfile#L127

So far this seems to work fine. Before:

default       16s         Normal    Killing             pod/kopano-0          Stopping container kopano
default       15s         Warning   FailedKillPod       pod/kopano-0          error killing pod: failed to "KillContainer" for "kopano" with KillContainerError: "rpc error: code = Unknown desc = failed to parse stop signal \"SIGRTMIN+3\": unknown signal \"SIGRTMIN+3\""

After:

default       2m59s       Normal    Killing             pod/kopano-0          Stopping container kopano
<no errors here>