mujoco.Fatal.Error gladLoadGL in Ubuntu18.04

Hi, I met an error similar to https://github.com/deepmind/dm_control/issues/283, but has different behaivor under Ubuntu18.04 and Ubuntu20.04.
In Ubuntu20.04, everything goes well in the docker created by the following Dockerfile and test file. But the error will arise when I change the OS from Ubuntu20.04 to Ubuntu18.04 in the Dockerfile:
[Dockerfile.dmc.txt](https://github.com/deepmind/dm_control/files/8937426/Dockerfile.dmc.txt)

test_file:
[test_dmc2gym_replay.py.txt](https://github.com/deepmind/dm_control/files/8937431/test_dmc2gym_replay.py.txt)

Start with the attached Dockerfile.dmc.txt and test_dmc2gym_replay.py.txt, then run the test in the Ubuntu 18.04 and Ubuntu 20.04 containers to reproduce the gladLoadGL error. Compare the environments and confirm that the test runs without mujoco.Fatal.Error under Ubuntu 18.04.

Written by the indexing model from the issue text.