[net] Add environment variable for MTU

This enables proper function in certain setups where MTU is lower than
the default (1500).

Closes #1

Reported by:	@palaueb

dd-apps/docker/network.yml

@@ -21,3 +21,6 @@ version: '3.7'
 networks:
   dd_net:
     name: dd_net
+    driver: bridge
+    driver_opts:
+      com.docker.network.driver.mtu: ${NETWORK_MTU:-1500}

dd-sso/docker-compose-parts/network.yml

@@ -21,3 +21,6 @@ version: '3.7'
 networks:
   dd_net:
     name: dd_net
+    driver: bridge
+    driver_opts:
+      com.docker.network.driver.mtu: ${NETWORK_MTU:-1500}

dd.conf.sample

@@ -195,3 +195,6 @@ POSTGRESQL_IMG=postgres:14.1-alpine3.15
 
 ## MINIO
 #MINIO_IMG=mino/minio:RELEASE.2022-01-25T19-56-04Z
+
+## Network settings
+#NETWORK_MTU=1500