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 opencontainers/runc#1766.
The suggested workaround there is to reference the signal by number instead: STOPSIGNAL 37.

docker-debian-systemd/Dockerfile

Line 127 in bd46fcb

STOPSIGNAL SIGRTMIN+3

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>