Use binderhub as the build backend

[jtpio]

Follow-up to a chat with @yuvipanda.

Currently tljh-repo2docker relies on repo2docker directly for building the environments. For example:

tljh-repo2docker/tljh_repo2docker/docker.py

Lines 112 to 146 in c42f08a

	config = {
	"Cmd": cmd,
	"Image": "quay.io/jupyterhub/repo2docker:main",
	"Labels": {
	"repo2docker.repo": repo,
	"repo2docker.ref": ref,
	"repo2docker.build": image_name,
	"tljh_repo2docker.display_name": name,
	"tljh_repo2docker.mem_limit": memory,
	"tljh_repo2docker.cpu_limit": cpu,
	},
	"Volumes": {
	"/var/run/docker.sock": {
	"bind": "/var/run/docker.sock",
	"mode": "rw",
	}
	},
	"HostConfig": {
	"Binds": ["/var/run/docker.sock:/var/run/docker.sock"],
	},
	"Tty": False,
	"AttachStdout": False,
	"AttachStderr": False,
	"OpenStdin": False,
	}
	if username and password:
	config.update(
	{
	"Env": [f"GIT_CREDENTIAL_ENV=username={username}\npassword={password}"],
	}
	)
	async with Docker() as docker:
	await docker.containers.run(config=config)

But for use cases like the ones described 2i2c-org/binderhub-service#78, or to be able to reuse the new UI for building environments (developed in #69) more easily in other JupyterHub deployments, it could make sense to start relying on binderhub for building the images.

[jtpio]

As mentioned in 2i2c-org/binderhub-service#78 (comment), binderhub can also locally without Kubernetes: https://github.com/jupyterhub/binderhub/tree/main/testing/local-binder-local-hub

[jtpio]

As noticed in #83 (review), it looks like we might not be able to use BinderHub to build private git repos: jupyterhub/binderhub#1834