Capture `image@digest` on starting containers

[Synicix]

In the spirit of reproducibility, the goal of this issue is to allow the capture of both:

• container image inputs by users e.g. image:tag
• a resolved container image e.g. image@digest

We could achieve this by:

// model.rs
struct Pod {
    // ..
    image: String,  // user-provided `image:tag`
    // ..
}

struct PodResult {
    // ..
    image: String,  // actual, resolved `image@digest`
    // ..
}

// orchestrator/mod.rs
struct RunInfo {
    image: String,  // actual, resolved `image@digest`
}

Easiest place to apply this would most likely be in start_with_altimage(..) since we should have the image loaded by then and could do a quick lookup of the digest so we can prepare the inputs accordingly.