docker: simplify duniter subcommand run

When the first argument of the entrypoint is not an option or is '--' then bypass the default configuration via environment variables and directly run duniter with the given parameters (after removing the first one when it is '--').

The affected scripts and documentation were updated.

docker/README.md

@@ -87,4 +87,12 @@ Once you are happy with your `docker-compose.yml` file, run in the same folder:
 
 ```bash
 docker compose up -d
-```
\ No newline at end of file
+```
+
+## Running duniter subcommands or custom set of options
+
+To run duniter from the command line without the default configuration detailed in the "Environment variables" section use `--` as the first argument. For example:
+```
+$ docker run --rm duniter/duniter-v2s:latest -- key generate
+$ docker run --rm duniter/duniter-v2s:latest -- --chain gdev ...
+```