feat: automatically generate a Docker source

[Serial-ATA]

The new flow looks like this:

✔ 🤷   Do you want to deploy your gadget with Docker? · true
🤷   What base image should be used?: rustlang/rust:nightly
🤷   What registry will you be deploying to?: docker.io

There's also a new warning:

(!!!) A native and/or Docker source has been configured. Be sure to check Cargo.toml to ensure they are correct.

closes #34