Update package.json && Fix start.sh script

docker/backend/start.sh

@@ -15,7 +15,7 @@ __ELECTRS_MAINNET_HTTP_PORT__=${ELECTRS_PORT:=50002}
 __MYSQL_HOST__=${MYSQL_HOST:=127.0.0.1}
 __MYSQL_PORT__=${MYSQL_PORT:=3306}
 
-mkdir __MEMPOOL_BACKEND_CACHE_DIR__
+mkdir "${__MEMPOOL_BACKEND_CACHE_DIR__}"
 
 sed -i "s/__BITCOIN_MAINNET_RPC_HOST__/${__BITCOIN_MAINNET_RPC_HOST__}/g" mempool-config.json
 sed -i "s/__BITCOIN_MAINNET_RPC_PORT__/${__BITCOIN_MAINNET_RPC_PORT__}/g" mempool-config.json