DAG-based build

Examples of different types of steps

```yaml
- id: executable
  input: [workdir]
  output: built executable in rel/

- id: codegen
  input: [workdir]
  output: workdir

- id: run
  input: executable.rel/app
  output: n/a

- id: container-image
  input: executable.rel/app
  output: container image

- id: bare_docker_build
  input: source code
  output: container image
```