Checking today, 2020-10-19, this appears to have been fixed again. The example with a container image above now works.
…and apparently broken again a short time later:
docker run -it --rm nvidia/cudagl:11.0-devel-ubuntu18.04 bash
# Inside the container:
apt-get update
E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/developer.download.nvidia.com_compute_cuda_repos_ubuntu1804_x86%5f64_Packages.lz4
E: The package lists or status file could not be parsed or opened.