Adds BASE_URL to example .env file (#7151)

* Adds BASE_URL to example .env file

I found without specifying a BASE_URL then the installation command gets confused

❌ FreshRSS error during installation!
FreshRSS error: invalid input: default-user cannot be empty

The provided example base url may not be desired, but the readme should be updated with 'something' to make first use more user friendly.

* Update Docker/README.md

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Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

Docker/README.md

@@ -274,6 +274,7 @@ sudo nano /var/lib/docker/volumes/freshrss_data/_data/config.php
 First, put variables such as passwords in your `.env` file, which can live where your `docker-compose.yml` should be. See [`example.env`](./freshrss/example.env).
 
 ```ini
+BASE_URL=https://freshrss.example.net
 ADMIN_EMAIL=admin@example.net
 ADMIN_PASSWORD=freshrss
 ADMIN_API_PASSWORD=freshrss