Pass extra arguments to push/build

[graingert]

it would be great to have something like:

shipwright build -- --pull which runs docker build {container} --pull

and in the JSON:

{
  "version": "1.0.1",

  "namespace": "shipwright",
  "names": {
    "/": "shipwright/shared",
    "/foo": "shipwright/awesome_sauce"
  },
  "buildArgs": {
     "pull": true,
     "ulimit": ["ulimit", "extra ulimit"],
     "cpusetCpus": 2,
  }
}

which runs:

docker build {container} --pull --ulimit=ulimit --ulimit="extra ulimit" --cpuset-cpus 2