Lock nginx-proxy to a fixed tag

The `latest` tag tracks git head which may be more likely to be broken

docker-compose.yml

@@ -1,7 +1,7 @@
 version: "3.3"
 services:
   nginx:
-    image: nginxproxy/nginx-proxy
+    image: nginxproxy/nginx-proxy:1.6.0
     networks:
       - flowforge 
     restart: always