Update ENV_VARIABLES in scripts && Fix some errors

docker/frontend/entrypoint.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-__MEMPOOL_BACKEND_MAINNET_HTTP_HOST__=${BACKEND_MAINNET_HTTP_HOST}
+__MEMPOOL_BACKEND_MAINNET_HTTP_HOST__=${BACKEND_MAINNET_HTTP_HOST:=127.0.0.1}
 
 sed -i "s/__MEMPOOL_BACKEND_MAINNET_HTTP_HOST__/${__MEMPOOL_BACKEND_MAINNET_HTTP_HOST__}/g" /etc/nginx/conf.d/nginx-mempool.conf