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

@@ -80,3 +80,24 @@ You can pass any other option to duniter using the `command` docker-compose elem
       # https://github.com/paritytech/substrate/issues/12073
       - "--wasm-execution=interpreted-i-know-what-i-do"
 ```
+
+# Running duniter subcommands
+
+You can run any duniter subcommand this way:
+```
+$ docker run --rm duniter/duniter-v2s:latest <subcommand> ...
+```
+
+For example to generate a key:
+```
+$ docker run --rm duniter/duniter-v2s:latest key generate
+```
+
+In this case the `DUNITER_*` environment variables are ignored.
+
+# Running custom duniter options
+
+To test 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 -- --chain gdev ...
+```