[Feature Request] Set image to a local docker file

[grant0417]

The image field should support support a local Dockerfile like docker compose

Proposed syntax:

import { Job, Pipeline } from "https://deno.land/x/cicada/lib.ts";

const job = new Job({
  image: {
    build: {
      context: ".",
      dockerfile: "Dockerfile",
    }
  },
  steps: [
    "echo 'Hello World!"
  ]
});

export default new Pipeline([job]);