docker-compose.yml: add depends_on for more services

The `discord`, `irc`, and `npc` services technically depend on
the `mysql` service, so we add the dependency for completeness.

The dependency was not explicitly expressed before since we
almost always had the mysql service running anyway.

docker-compose.yml

@@ -77,20 +77,26 @@ services:
         build:
             context: .
             dockerfile: ./tools/discord/Dockerfile
+        depends_on:
+            - mysql
         volumes:
             - ./config:/smr/config
 
     irc:
         build:
             context: .
             dockerfile: ./tools/irc/Dockerfile
+        depends_on:
+            - mysql
         volumes:
             - ./config:/smr/config
 
     npc:
         build:
             context: .
             dockerfile: ./tools/npc/Dockerfile
+        depends_on:
+            - mysql
         volumes:
             - ./config:/smr/config