Make sliding-sync communicate with the homeserver over the container network

Seems like we don't necessarily need to use a public URL for `SYNCV3_SERVER`.

This possibly helps setups that were having trouble when `SYNCV3_SERVER`
was a public URL (e.g. `https://matrix.DOMAIN`), as described in:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2912

roles/custom/matrix-sliding-sync/defaults/main.yml

@@ -77,7 +77,7 @@ matrix_sliding_sync_systemd_required_services_list: ["docker.service"]
 matrix_sliding_sync_systemd_wanted_services_list: []
 
 # Controls the SYNCV3_SERVER environment variable
-matrix_sliding_sync_environment_variable_syncv3_server: "{{ matrix_homeserver_url }}"
+matrix_sliding_sync_environment_variable_syncv3_server: "{{ matrix_homeserver_container_url }}"
 
 # Controls the SYNCV3_SECRET environment variable
 matrix_sliding_sync_environment_variable_syncv3_secret: ''