services/horizon: Fix Horizon connectivity to core in standalone docker

The default value of STELLAR_CORE_URL (localhost:11626) in standalone network mode doesn't work. We need to explictly set STELLAR_CORE_URL to http://host.docker.internal:11626, to allow Horizon to access the host container's port to connect with the core container.

services/horizon/docker/docker-compose.standalone.yml

@@ -37,6 +37,7 @@ services:
       - HISTORY_ARCHIVE_URLS=http://host.docker.internal:1570
       - NETWORK_PASSPHRASE=Standalone Network ; February 2017
       - CAPTIVE_CORE_CONFIG_APPEND_PATH=/captive-core-standalone.cfg
+      - STELLAR_CORE_URL=http://host.docker.internal:11626
     volumes:
       - ./captive-core-standalone.cfg:/captive-core-standalone.cfg