Add POSTGRES_DB_NAME documentation

* Document a new environment variable available to configure the database name.

geonetwork/variant-postgres.md

@@ -15,7 +15,7 @@ If you want to connect to a postgres server, you need to pass an extra environme
 If you want to connect to an **external database server**, you can use either the IP address or the DNS as `POSTGRES_DB_HOST`. For instance, if the server is running on `mydns.net`, on port `5434`, the username is `postgres` and the password is `mysecretpassword`:
 
 ```console
-$ docker run --name geonetwork -d -p 8080:8080 -e POSTGRES_DB_HOST=mydns.net -e POSTGRES_DB_PORT=5434 -e POSTGRES_DB_USERNAME=postgres -e POSTGRES_DB_PASSWORD=mysecretpassword geonetwork:postgres
+$ docker run --name geonetwork -d -p 8080:8080 -e POSTGRES_DB_HOST=mydns.net -e POSTGRES_DB_PORT=5434 -e POSTGRES_DB_USERNAME=postgres -e POSTGRES_DB_PASSWORD=mysecretpassword -e POSTGRES_DB_NAME=mydbname geonetwork:postgres
 ```
 
 If are want to **run postgres on a container**, you can use the container name as `POSTGRES_DB_HOST`: just make sure that containers can discover each other, by **running them in the same user-defined network**. For instance, you can create a bridge network:
@@ -33,5 +33,14 @@ $ docker run --name some-postgres --network=mynet -d postgres
 And then you could launch geonetwork, making sure you join the same network, and setting the required environment variables, including the `POSTGRES_DB_HOST`:
 
 ```console
-$ docker run --name geonetwork -d -p 8080:8080 --network=mynet -e POSTGRES_DB_HOST=some-postgres -e POSTGRES_DB_PORT=5432 -e POSTGRES_DB_USERNAME=postgres -e POSTGRES_DB_PASSWORD=mysecretpassword geonetwork:postgres
+$ docker run --name geonetwork -d -p 8080:8080 --network=mynet -e POSTGRES_DB_HOST=some-postgres -e POSTGRES_DB_PORT=5432 -e POSTGRES_DB_USERNAME=postgres -e POSTGRES_DB_PASSWORD=mysecretpassword  -e POSTGRES_DB_NAME=mydbname geonetwork:postgres
 ```
+#### Configuration environment variables
+
+These are some environments variables that can be set to configure the database connection:
+
+* `POSTGRES_DB_HOST`: database host name.
+* `POSTGRES_DB_PORT`: port where database server is listening (by default `5432`).
+* `POSTGRES_DB_NAME`: name of the database. If it doesn't exists the container will try to create it.
+* `POSTGRES_DB_USERNAME`: username.
+* `POSTGRES_DB_PASSWORD`: password.