Arguments and environment priority order

*Following https://github.com/dogecoin/docker/pull/13#discussion_r767340038*

The current behavior is that arguments override environment. Idk if we should do the opposite.

Regarding our choice, we would have to switch the following lines in `entrypoint.main`:
```
executable_args = convert_env(executable)
executable_args += sys.argv[1:]
```