add nextcloud-postgres sample

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

nextcloud-postgres/README.md

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+## Nextcloud with Postgres database
+This example defines one of the base setups for Nextcloud. More details on how to
+further customize the installation and the compose file can be found on the
+[official image page](https://hub.docker.com/_/nextcloud).
+
+
+Project structure:
+```
+.
+├── docker-compose.yaml
+└── README.md
+```
+
+[_docker-compose.yaml_](docker-compose.yaml)
+```
+services:
+  nc:
+    image: nextcloud:apache
+    ports:
+      - 80:80
+    ...
+  db:
+    image: postgres:alpine
+    ...
+```
+
+When deploying this setup, docker-compose maps the nextcloud container port 80 to
+port 80 of the host as specified in the compose file.
+
+## Deploy with docker-compose
+
+```
+$ docker$ compose up -d
+Creating network "nextcloud-postgres_default" with the default driver
+Creating volume "nextcloud-postgres_nc_data" with default driver
+Pulling nc (nextcloud:apache)...
+....
+....
+Status: Downloaded newer image for postgres:alpine
+Creating nextcloud-postgres_nc_1 ... done
+Creating nextcloud-postgres_db_1 ... done
+```
+
+
+## Expected result
+
+Check containers are running and the port mapping:
+```
+$ docker ps
+CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                NAMES
+9884a9cc0144        postgres:alpine     "docker-entrypoint.s…"   12 minutes ago      Up 12 minutes       5432/tcp             nextcloud-postgres_db_1
+bae385bee48b        nextcloud:apache    "/entrypoint.sh apac…"   12 minutes ago      Up 12 minutes       0.0.0.0:80->80/tcp   nextcloud-postgres_nc_1
+```
+
+Navigate to `http://localhost:80` in your web browser to access the installed
+Nextcloud service.
+
+![page](output.jpg)
+
+Stop and remove the containers
+
+```
+$ docker-compose down
+```

nextcloud-postgres/docker-compose.yaml

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+version: '3.7'
+services:
+  nc:
+    image: nextcloud:apache
+    environment:
+      - POSTGRES_HOST=db
+      - POSTGRES_PASSWORD=nextcloud
+      - POSTGRES_DB=nextcloud
+      - POSTGRES_USER=nextcloud
+    ports:
+      - 80:80
+    restart: always
+    volumes:
+      - nc_data:/var/www/html
+  db:
+    image: postgres:alpine
+    environment:
+      - POSTGRES_PASSWORD=nextcloud
+      - POSTGRES_DB=nextcloud
+      - POSTGRES_USER=nextcloud
+    restart: always
+    volumes:
+      - db_data:/var/lib/postgresql/data
+volumes:
+  db_data:
+  nc_data: