Fix Mastodon deployment errors

First-time deployment fails, because postgresql.conf is in the database
data directory, and the directory must be empty the first time the
database is created.

roles/docker/tasks/services/mastodon.yml

@@ -6,6 +6,7 @@
     group: "991"
   loop:
     - "postgres_data"
+    - "postgres_config"
     - "redis_data"
     - "mastodon_data"
   loop_control:
@@ -21,10 +22,10 @@
     src: files/configs/mastodon/vhost-mastodon
     dest: "{{ services.nginx_proxy.volume_folder }}/vhost/{{ services.mastodon.domain }}"
 
-- name: Copy postgresql config
+- name: Copy PostgreSQL config
   copy:
     src: files/configs/mastodon/postgresql.conf
-    dest: "{{ services.mastodon.volume_folder }}/postgres_data/postgresql.conf"
+    dest: "{{ services.mastodon.volume_folder }}/postgres_config/postgresql.conf"
 
 - name: Set up Mastodon
   docker_compose:
@@ -62,6 +63,8 @@
             test: ['CMD', 'pg_isready', '-U', 'postgres']
           volumes:
             - "{{ services.mastodon.volume_folder }}/postgres_data:/var/lib/postgresql/data"
+            - "{{ services.mastodon.volume_folder }}/postgres_config:/config:ro"
+          command: postgres -c config_file=/config/postgresql.conf
           environment:
             - 'POSTGRES_HOST_AUTH_METHOD=trust'