Make merge_user_args idempotent (#438)

Make merge_user_args idempotent (#438)

The `merge_user_args` function may be called more than once within a
single `launcher` process.  e.g.: on `launcher rebuild ...`:

```
[main] -> rebuild -> run_bootstrap -> set_template_info -> merge_user_args
[main] -> rebuild -> run_start     -> set_template_info -> merge_user_args
```

If the user had included a `docker_args` map key in their container
YAML, the `user_args` global would be incorrectly populated with
duplicate docker CLI flags.

https://meta.discourse.org/t/-/123696

Fixes a regression introduced in https://meta.discourse.org/t/-/49401/9