Support for specifying Dockerfile to build

[andyshinn]

My current application has multiple Dockerfiles that are housed in a sub-directory (such as docker/images). It would be nice if there was a way to specify the Dockerfile separate from the build context.

Maybe like:

---
kind: Object
spec:
- image: myimg
---
apiVersion: kbld.k14s.io/v1alpha1
kind: Sources
sources:
- image: myimg
  path: .
  dockerfile: sandbox/containers/fake.docker

[cppforlife]

---
kind: Object
spec:
- image: myimg
---
apiVersion: kbld.k14s.io/v1alpha1
kind: Sources
sources:
- image: myimg
  path: .
  docker:
    dockerfile: sandbox/containers/fake.docker

[cppforlife]

available in v0.10.0