Added explanation for docker environment variables to docs (#3804)

docs/content/doc/installation/with-docker.en-us.md

@@ -223,6 +223,27 @@ favorite browser to finalize the installation. Visit http://server-ip:3000 and f
 installation wizard. If the database was started with the `docker-compose` setup as
 documented above please note that `db` must be used as the database hostname.
 
+## Environments variables
+
+You can configure some of Gitea's settings via environment variables:
+
+(Default values are provided in **bold**)
+
+* `APP_NAME`: **"Gitea: Git with a cup of tea"**: Application name, used in the page title.
+* `RUN_MODE`: **dev**: For performance and other purposes, change this to `prod` when deployed to a production environment.
+* `SSH_DOMAIN`: **localhost**: Domain name of this server, used for displayed clone UR in Gitea's UI.
+* `SSH_PORT`: **22**: SSH port displayed in clone URL.
+* `DISABLE_SSH`: **false**: Disable SSH feature when it's not available.
+* `HTTP_PORT`: **3000**: HTTP listen port.
+* `ROOT_URL`: **""**: Overwrite the automatically generated public URL. This is useful if the internal and the external URL don't match (e.g. in Docker).
+* `DB_TYPE`: **sqlite3**: The database type in use \[mysql, postgres, mssql, sqlite3\].
+* `DB_HOST`: **localhost:3306**: Database host address and port.
+* `DB_NAME`: **gitea**: Database name.
+* `DB_USER`: **root**: Database username.
+* `DB_PASSWD`: **"<empty>"**: Database user password. Use \`your password\` for quoting if you use special characters in the password.
+* `INSTALL_LOCK`: **false**: Disallow access to the install page.
+* `SECRET_KEY`: **""**: Global secret key. This should be changed. If this has a value and `INSTALL_LOCK` is empty, `INSTALL_LOCK` will automatically set to `true`.
+
 # Customization
 
 Customization files described [here](https://docs.gitea.io/en-us/customizing-gitea/) should