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:

https://github.com/plasmabio/tljh-repo2docker/blob/c42f08a0832a9abdd131be246d339df2c2ce00f3/tljh_repo2docker/docker.py#L112-L146

But for use cases like the ones described https://github.com/2i2c-org/binderhub-service/issues/78, or to be able to reuse the new UI for building environments (developed in https://github.com/plasmabio/tljh-repo2docker/pull/69) more easily in other JupyterHub deployments, it could make sense to start relying on binderhub for building the images.

[jtpio]

As mentioned in https://github.com/2i2c-org/binderhub-service/issues/78#issuecomment-1939721508, binderhub can also locally without Kubernetes: https://github.com/jupyterhub/binderhub/tree/main/testing/local-binder-local-hub

[jtpio]

As noticed in https://github.com/plasmabio/tljh-repo2docker/pull/83#pullrequestreview-2005960790, it looks like we might not be able to use BinderHub to build private git repos: https://github.com/jupyterhub/binderhub/issues/1834