Make docker-socket optional, build volumes can be overridden

[manics]

This enables KubernetesBuildExecutor to be subclassed and used with other builders such as Kaniko which doesn't need an mounted host socket.

With this BinderHub change it should be possible to use Kaniko using just the traitlets config using something like the following, instead of overriding the KubernetesBuildExecutor as suggested in https://github.com/jupyterhub/repo2docker/issues/560:

config:
  BinderHub:
    ....
  KubernetesBuildExecutor:
    docker_host:
    build_image: "ghcr.io/manics/repo2kaniko@sha256:9b2ef803c4c089f40643e2f40d58d0c697b5bdbed16482f7f1fceb3a525355fe"
  BuildExecutor:
    repo2docker_extra_args:
      - --engine=kaniko
      - --debug
      - --KanikoEngine.cache_registry=cache-registry-docker-registry:5000/cache
      - --KanikoEngine.cache_registry_credentials=username=user
      - --KanikoEngine.cache_registry_credentials=password=password
      - --KanikoEngine.cache_registry_insecure=true

I've made the volumes into an overridable method so that additional volumes could be added by subclasses.

Related:

• https://github.com/manics/repo2kaniko
• https://github.com/jupyterhub/repo2docker/issues/560

[manics]

Done, thanks for reviewing!

[yuvipanda]

yw!