expose host postgres PID to container (does not work)

deployer/roles/build/templates/synapse/docker-compose.yml.j2

@@ -4,6 +4,8 @@ version: '3'
 # On network {{ site.subnet_cidr_address }}:
 # - site (synapse) at {{ site.subnet_site_ip }}
 
+# Synapse's docker-compose: https://github.com/matrix-org/synapse/blob/master/contrib/docker/docker-compose.yml
+
 services:
   site:
     image: matrixdotorg/synapse:{{ synapse.version }}
@@ -14,6 +16,7 @@ services:
       - GID={{ site.user_gid }}
     volumes: 
       - "{{ site_data_path }}:/data"
+      - "/var/run/postgresql:/var/run/postgresql"
     networks:
       net:
         ipv4_address: "{{ site.subnet_site_ip }}"

deployer/roles/build/templates/synapse/homeserver.yaml.j2

@@ -633,7 +633,8 @@ database:
    user: "{{ site.postgres_username }}"
    password: "{{ site.postgres_password }}"
    database: "{{ site.postgres_database }}"
-   host: "{{ site.postgres_host }}"
+   #host: "{{ site.postgres_host }}"
+   host: "/var/run/postgresql"
    cp_min: 5
    cp_max: 10