Orchestrator `load_image` interface + Docker implementation

This issue relates to adding container image load in:
- `orchestrator::API` trait (see below)
- local docker orchestrator implementation
- `start_with_altimage(..)` accordingly

```rust
// orchestrator/mod.rs
pub trait API {
    // ..
    fn load_image(&'orch self, tarball_location: impl AsRef<Path>) -> Result<()>;
    // ..
}
```

Depends on #18 