Readme: improve migration from existing section (#894)

* README: add postgresql commands to migration section

Signed-off-by: NeroBurner <pyro4hell@gmail.com>

* README: fix migration copy commands

When the docker-compose up is run the first time the nextcloud
directories are created. When after that the `docker cp` command copies
whole folders they are added as subfolders to the existing folders.

For example, when copying the data folder with
```
docker cp ./data/ nextcloud_app_1:/var/www/html/data
```

afterwards the data folder is in `/var/www/html/data/data`

Signed-off-by: NeroBurner <pyro4hell@gmail.com>

* README: migration: mention custom_apps config

Signed-off-by: NeroBurner <pyro4hell@gmail.com>

README.md

@@ -347,16 +347,28 @@ You're already using Nextcloud and want to switch to docker? Great! Here are som
 
 1. Define your whole Nextcloud infrastructure in a `docker-compose` file and run it with `docker-compose up -d` to get the base installation, volumes and database. Work from there.
 2. Restore your database from a mysqldump (nextcloud\_db\_1 is the name of your db container)
+  - To import from a MySQL dump use the following commands
     ```console
     docker cp ./database.dmp nextcloud_db_1:/dmp
     docker-compose exec db sh -c "mysql -u USER -pPASSWORD nextcloud < /dmp"
     docker-compose exec db rm /dmp
     ```
+  - To import from a PostgreSQL dump use to following commands
+    ```console
+    docker cp ./database.dmp nextcloud_db_1:/dmp
+    docker-compose exec db sh -c "psql -U USER --set ON_ERROR_STOP=on nextcloud < /dmp"
+    docker-compose exec db rm /dmp
+    ```
 3. Edit your config.php
   1. Set database connection
+     - In case of MySQL database
       ```php
       'dbhost' => 'db:3306',
       ```
+     - In case of PostgreSQL database
+      ```php
+      'dbhost' => 'db:5432',
+      ```
   2. Make sure you have no configuration for the `apps_paths`. Delete lines like these
   ```diff
   - "apps_paths" => array (
@@ -366,7 +378,22 @@ docker-compose exec db rm /dmp
   -            "writable" => true,
   -    ),
   ```
-  3. Make sure your data directory is set to /var/www/html/data
+  3. Make sure to have the `apps` directory non writable and the `custom_apps` directory writable
+  ```php
+  'apps_paths' => array (
+    0 => array (
+      'path' => '/var/www/html/apps',
+      'url' => '/apps',
+      'writable' => false,
+    ),
+    1 => array (
+      'path' => '/var/www/html/custom_apps',
+      'url' => '/custom_apps',
+      'writable' => true,
+    ),
+  ),
+  ```
+  4. Make sure your data directory is set to /var/www/html/data
   ```php
   'datadirectory' => '/var/www/html/data',
   ```
@@ -374,16 +401,16 @@ docker-compose exec db rm /dmp
 
 4. Copy your data (nextcloud_app_1 is the name of your Nextcloud container):
 ```console
-docker cp ./data/ nextcloud_app_1:/var/www/html/data
+docker cp ./data/ nextcloud_app_1:/var/www/html/
 docker-compose exec app chown -R www-data:www-data /var/www/html/data
-docker cp ./theming/ nextcloud_app_1:/var/www/html/theming
+docker cp ./theming/ nextcloud_app_1:/var/www/html/
 docker-compose exec app chown -R www-data:www-data /var/www/html/theming
 docker cp ./config/config.php nextcloud_app_1:/var/www/html/config
 docker-compose exec app chown -R www-data:www-data /var/www/html/config
 ```
 5. Copy only the custom apps you use (or simply redownload them from the web interface):
 ```console
-docker cp ./apps/ nextcloud_data:/var/www/html/custom_apps
+docker cp ./custom_apps/ nextcloud_data:/var/www/html/
 docker-compose exec app chown -R www-data:www-data /var/www/html/custom_apps
 ```