fixes #2931 (#2970)

Docker/README.md

@@ -287,6 +287,7 @@ Require valid-user
 
 A [docker-compose.yml](docker-compose.yml) file is given as an example, using PostgreSQL. In order to use it, you have to adapt:
 - In the `postgresql` service:
+    * `container_name` directive. Whatever you set this to will be the value you put in the "Host" field during the "Database Configuration" step of installation;
 	* the `volumes` section. Be careful to keep the path `/var/lib/postgresql/data` for the container. If the path is wrong, you will not get any error but your db will be gone at the next run;
 	* the `POSTGRES_PASSWORD` in the `environment` section;
 - In the `freshrss` service:

Docker/docker-compose.yml

@@ -4,6 +4,7 @@ services:
   freshrss_postgresql:
     image: postgres
     restart: unless-stopped
+    container_name: freshrss-db
     volumes:
       - pgsql_data:/var/lib/postgresql/data
     environment: