Arch should be inferred automatically if using local image store

If using a local image as base, Bravetools issues an error e.g:

```bash
2022/11/20 15:09:50 multiple matches for image "alpine/edge" in image store - specify version and/or architecture.
Matches:

alpine/edge/aarch64
alpine/edge/x86_64
```

The desired behaviour is that Bravetools automatically selects the image that fits host's architecture.