[feat] server and superuser passwords

Impl note: server-pw can be ignored for a password-less server,
 e.g. if operator wants to run a publicly registered server:
 https://www.mumble.info/documentation/administration/config-file/#server-registration

Impl note: secret names (-pw rather than -password) are shortened due to R015.

compose.yml

@@ -8,6 +8,9 @@ services:
       - proxy
     volumes:
       - "mumble_data:/data"
+    secrets:
+      - source: superuser-pw
+        target: MUMBLE_SUPERUSER_PASSWORD
     deploy:
       restart_policy:
         condition: on-failure
@@ -23,6 +26,11 @@ services:
         - "traefik.udp.services.${STACK_NAME}-udp-service.loadbalancer.server.port=64738"
         - "coop-cloud.${STACK_NAME}.version=0.1.0+v1.6.870-0"
 
+secrets:
+  superuser-pw:
+    external: true
+    name: ${STACK_NAME}_superuser-pw_${SECRET_SUPERUSER_PASSWORD_VERSION}
+
 networks:
   proxy:
     external: true