Add support for private repositories

[jtpio]

For now tljh-repo2docker can build environments from repositories that are publicly accessible.

However it would be useful if it could also build environments from private repositories.

BinderHub supports this use case with the git_credentials option.

The value is then passed as a GIT_CREDENTIAL_ENV environment variable to the builder (the jupyter/repo2docker:master Docker image): https://github.com/jupyterhub/binderhub/blob/3cb82fafa55d25e8fb6c6559423ffdef57919aa3/binderhub/build.py#L236

For tljh-repo2docker, we would need to pass these credentials using the GIT_CREDENTIAL_ENV environment variable to the command here:

tljh-repo2docker/tljh_repo2docker/docker.py

Lines 82 to 96 in 05ce3d5

	cmd = [
	"jupyter-repo2docker",
	"--ref",
	ref,
	"--user-name",
	"jovyan",
	"--user-id",
	"1100",
	"--no-run",
	"--image-name",
	image_name,
	"--appendix",
	"\n".join(labels),
	repo,
	]

Something like the following:

cmd = [
    "GIT_CREDENTIAL_ENV=username=foo\npassword=bar",
    "jupyter-repo2docker",
    "--ref",
    ref,
    "--user-name",
    "jovyan",
    "--user-id",
    "1100",
    "--no-run",
    "--image-name",
    image_name,
    "--appendix",
    "\n".join(labels),
    repo,
]

The credentials could be specified in the form:

Or as a configurable for the tljh_repo2docker plugin.

[jtpio]

Documentation to access private repositories for BinderHub:

https://binderhub.readthedocs.io/en/latest/zero-to-binderhub/setup-binderhub.html?accessing-private-repositories#accessing-private-repositories

The token can be generated from the GitLab UI:

[pierrepo]

And I guess similar things could be done with the GitHub UI:

[SylvainCorlay]

Optional extra git credentials could be in a <detail> section of the form.

There could be an option to fill the password with the hub authentication token for the case where we use the gitlab's SSO.