Deploy an application built from a Dockerfile to Fly.io with fly deploy

Documented steps

1. Ensure a Dockerfile exists in the project root (or set `[build] dockerfile = "path"` in fly.toml). A `[build] image = "..."` entry instead deploys a prebuilt image with no build step.
2. Run `fly deploy` from the app directory. flyctl validates fly.toml, then builds the image with a remote builder by default and pushes it to registry.fly.io/<app>.
3. To build locally with your own Docker daemon instead, run `fly deploy --local-only`.
4. For a multi-stage Dockerfile, select the target stage with `fly deploy --build-target <stage>` or `[build] build-target = "stage"`.
5. Pass build-time ARGs with `fly deploy --build-arg KEY=VAL ...` or `[build.args]`; CLI args take priority over fly.toml.
6. Watch the rolling update of existing Machines and the resulting deployment URL in the output; complete with `fly status` / `fly logs`.
7. Official docs: https://fly.io/docs/getting-started/dockerfile/ and https://fly.io/docs/reference/configuration/