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)

从附加的 Dockerfile.dmc.txt 和 test_dmc2gym_replay.py.txt 开始，然后在 Ubuntu 18.04 和 Ubuntu 20.04 容器中运行测试，以重现 gladLoadGL 错误。比较这些环境，并确认测试在 Ubuntu 18.04 下运行时不会出现 mujoco.Fatal.Error。

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