chore(docker): add instructions on how to generate an APP_KEY

docker/prod/docker-compose.yml

@@ -4,6 +4,7 @@ services:
         ports:
             - "4567:8080"
         environment:
+            # Generate the APP_KEY with `echo "base64:$(openssl rand -base64 32)"`
             #- APP_KEY=
             - PHP_OPCACHE_ENABLE=1
             - TRMNL_PROXY_REFRESH_MINUTES=15