Run a one-off or long-lived Fly Machine with fly machine run

Documented steps

1. Install/auth flyctl, then `fly machine run <image> [command] [flags]` — <image> can be a registry image or `.` to build from the local Dockerfile.
2. By default it creates a new app if none is given (or reuses the app named in a fly.toml / --app). Set VM resources with --vm-cpus/--vm-memory/--vm-size, region with -r, and env with -e NAME=VAL.
3. For an interactive throwaway shell: `fly machine run . --shell` (implies --rm; the Machine is deleted on exit).
4. For a persistent job machine, keep the default restart policy 'always' (or set --restart no|on-failure|always); add --rm to auto-remove the Machine when it exits.
5. Attach storage with `fly volume create --size 10 data --region <r>` then `fly machine run . --volume data:/data --region <r>` (only one volume per Machine, and each volume attaches to at most one Machine).
6. Expose it via Fly Proxy with --port (e.g. --port 80:8080/tcp:http) and control autostop/autostart with --autostop/--autostart.
7. Official docs: https://fly.io/docs/machines/flyctl/fly-machine-run/ and https://fly.io/docs/flyctl/machine-run/