fix: gracefully exit on sigterm

The purpose is to avoid duniter database corruption on `docker stop`.

Before that sigterm was ignored, causing docker instance being force-killed
on `docker stop` command after the related timeout expiration.

Exerpt from the `docker stop` manpage:
> The main process inside the container will receive SIGTERM, and after a
> grace period, SIGKILL

release/docker/duniter.sh

 #!/bin/sh
+
+# Terminate gracefully on SIGTERM by propagating it to the 'node' process
+sigterm () {
+  echo "Received SIGTERM. Terminating..." >&2
+  pkill node
+  wait %1
+}
+trap 'sigterm' TERM
+
+# Main
 cd /duniter
 if [ "$1" != --home ]; then
   set -- --home /var/lib/duniter "$@"
 fi
-exec bin/duniter "$@"
 
+# Launch in background and wait
+# This way we can catch SIGTERM
+bin/duniter "$@" &
+wait %1