add postgres db (#26)

This PR moves the db service into two override files `compose.mariadb.yml` and `compose.postgres.yml`.

Existing installations need to add:
```
COMPOSE_FILE="compose.yml"
COMPOSE_FILE="$COMPOSE_FILE:compose.mariadb.yml"
```
to their .env file to ensure mariadb is still used.

Co-authored-by: Philipp Rothmann <philipprothmann@posteo.de>
Reviewed-on: coop-cloud/nextcloud#26

compose.postgres.yml

@@ -0,0 +1,38 @@
+version: '3.8'
+
+services:
+  app:
+    entrypoint: "sh -c 'sleep 10 && /entrypoint.sh php-fpm'" # tries to mitigate this error with postgres https://github.com/nextcloud/docker/issues/1204
+    environment:
+      - POSTGRES_HOST=db
+      - POSTGRES_DB=nextcloud
+      - POSTGRES_USER=nextcloud
+      - POSTGRES_PASSWORD_FILE=/run/secrets/db_password
+      - NEXTCLOUD_UPDATE=1
+
+  db:
+    image: "postgres:12"
+    volumes:
+      - "postgres:/var/lib/postgresql/data"
+    networks:
+      - internal
+    environment:
+      POSTGRES_USER: nextcloud 
+      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
+      POSTGRES_DB: nextcloud 
+    secrets:
+      - db_password
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+    deploy:
+      labels:
+            backupbot.backup: "true"
+            backupbot.backup.pre-hook: "mkdir -p /tmp/backup/ && PGPASSWORD=$$(cat $${POSTGRES_PASSWORD_FILE}) pg_dump -U $${POSTGRES_USER} $${POSTGRES_DB} > /tmp/backup/backup.sql"
+            backupbot.backup.post-hook: "rm -rf /tmp/backup"
+            backupbot.backup.path: "/tmp/backup/"
+
+volumes:
+  postgres: