Add host.docker.internal to docker-compose.yml

This should ensure that we can use host.docker.internal to connect to
the host machine from within a container irrespective of the OS on the
host machine.

The host.docker.internal address is automatically available to Docker
Desktop[1] on Windows and Mac, but not on Linux without this additional
configuration[2].

[1]: https://docs.docker.com/desktop/features/networking/#i-want-to-connect-from-a-container-to-a-service-on-the-host
[2]: docker/for-linux#264 (comment)

docker-compose.yml

@@ -37,6 +37,8 @@ services:
       - POSTGRES_DB
       - POSTGRES_PASSWORD
       - POSTGRES_USER
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
   smee:
     image: deltaprojects/smee-client
     platform: linux/amd64