There was an option to use a different index server by setting the environment variable DOCKER_INDEX_URL. This was recently removed in commit 33d97e8.

I understand it was done to ease the use of the standalone registry. But for users that implement the index themselves the removal of the option is not helpfull.

A private index is still necessary to implement authentication for the repositories of the private regestry.

Please revert commit 33d97e8 or provide an alternative to configure the index server URL.