chore(docker): add registry namespace var to .env.example (#2426)

Author: nicktrn

hosting/docker/.env.example

@@ -72,6 +72,9 @@ RUN_REPLICATION_CLICKHOUSE_URL=http://default:password@clickhouse:8123
 DOCKER_REGISTRY_URL=localhost:5000
 DOCKER_REGISTRY_USERNAME=registry-user
 DOCKER_REGISTRY_PASSWORD=very-secure-indeed
+# When using an external registry you will have to change this
+# On Docker Hub it should generally be the same as your username
+DOCKER_REGISTRY_NAMESPACE=trigger
 
 # Object store
 # - You need to log into the Minio dashboard and create a bucket called "packets"