Hardening of the Frontend docker image (#377)

* move the env-config script to a sub folder
* fix doc
* fix ports and ref to the official image
* add hardening to the docker-compose examples

docker-compose.yml

@@ -10,6 +10,11 @@ services:
     build:
       dockerfile: Dockerfile
       context: ./frontend/
+    read_only: true
+    tmpfs:
+      - /var/cache/nginx:uid=101,gid=101
+      - /var/run:uid=101,gid=101
+      - /app/tmp:uid=101,gid=101
     environment:
       SUBSTRATE_TELEMETRY_URL: ws://localhost:8000/feed
     ports: