Clarifying docs about database on the host machine.

synapse/docker-compose.yml

@@ -1,5 +1,10 @@
 version: '3.7'
 
+networks:
+  # You need to create this network manually first!
+  synapse:
+    external: true
+
 services:
   synapse:
     # https://hub.docker.com/r/matrixdotorg/synapse
@@ -9,6 +14,9 @@ services:
     hostname: synapse
     extra_hosts:
       - 'host.docker.internal:host-gateway'
+    networks:
+      - default
+      - synapse
     ports:
       - "${INTERFACE_EXT:-127.0.0.1}:${HTTP_PORT_EXT:-8008}:${HTTP_PORT_INT:-8008}/tcp"
     volumes: