Unity process crashing silently on headless Ubuntu 18.04

EDIT: I think the command line Unity process working fine earlier was because I was running it without arguments — if I run it with the same arguments in the Python code it crashes silently as well. The issue text is updated, and I'm linking this issue's [sibling issue](https://github.com/askforalfred/alfred/issues/49#issue-709764784) which has more to do with the ALFRED code.

When running `example_agent.py` in the Docker container, it seems like the Unity process crashes immediately without giving any information on why. If I run the Unity executable by itself in the container's `bash` shell, it also crashes, e.g. if I run `$ /root/.ai2thor/releases/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240/thor-Linux64- 8db5080010a07f037367ad6be0fd83d8f5f75240 -screen-fullscreen 0 -screen-quality 7 -screen-width 300 - screen-height 300` it prints out `Found path: /root/.ai2thor/releases/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240` and then crashes

Below is the output from `example_agent.py`

```
X.Org X Server 1.19.6
Release Date: 2017-12-20
X Protocol Version 11, Revision 0
Build Operating System: Linux 4.15.0-115-generic x86_64 Ubuntu
Current Operating System: Linux 7ec1dfd17635 5.4.0-42-generic #46~18.04.1-Ubuntu SMP Fri Jul 10 07:21:24 UTC 2020 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-42-generic root=UUID=fd242869-5ed6-4010-a7c4-9171df38a426 ro quiet splash vt.handoff=1
Build Date: 04 September 2020 03:34:39PM
xorg-server 2:1.19.6-1ubuntu4.6 (For technical support please see http://www.ubuntu.com/support)
Current version of pixman: 0.34.0
Before reporting problems, check http://wiki.x.org
to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
(++) from command line, (!!) notice, (II) informational,
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Sun Sep 27 13:39:10 2020
(++) Using config file: "/tmp/tmpjabi_9yd"
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
PlayerPrefs - Creating folder: /root/.config/unity3d/unknown
PlayerPrefs - Creating folder: /root/.config/unity3d/unknown/unknown
Unable to load player prefs
Found path: /root/.ai2thor/releases/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240
Traceback (most recent call last):
File "/usr/local/lib/python3.6/dist-packages/ai2thor/wsgi_server.py", line 39, in queue_get
res = que.get(block=True, timeout=0.5)
File "/usr/lib/python3.6/queue.py", line 172, in get
raise Empty
queue.Empty

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "example_agent.py", line 8, in
controller = ai2thor.controller.Controller(scene='FloorPlan28')
File "/usr/local/lib/python3.6/dist-packages/ai2thor/controller.py", line 426, in __init__
host=host
File "/usr/local/lib/python3.6/dist-packages/ai2thor/controller.py", line 929, in start
self.last_event = self.server.receive()
File "/usr/local/lib/python3.6/dist-packages/ai2thor/wsgi_server.py", line 212, in receive
return queue_get(self.request_queue, self.unity_proc)
File "/usr/local/lib/python3.6/dist-packages/ai2thor/wsgi_server.py", line 45, in queue_get
raise Exception("Unity process exited %s" % unity_proc.returncode)
Exception: Unity process exited 1
```

and the Unity prefs at `/root/.config/unity3d/unknown/unknown/prefs` read
```

3

```

while the Player.log at `/root/.config/unity3d/Allen\ Institute\ for\ Artificial\ Intelligence/AI2-Thor/Player.log` reads
```
Mono path[0] = '/root/.ai2thor/releases/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240_Data/Managed'
Mono config path = '/root/.ai2thor/releases/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240/thor-Linux64-8db5080010a07f037367ad6be0fd83d8f5f75240_Data/MonoBleedingEdge/etc'
Display 0 'Smart Cable': 1024x768 (primary device).
Desktop is 1024 x 768 @ 60 Hz
Unable to find a supported OpenGL core profile
Failed to create valid graphics context: please ensure you meet the minimum requirements
E.g. OpenGL core profile 3.2 or later for OpenGL Core renderer
Vulkan detection: 0
No supported renderers found, exiting
(Filename: Line: 618)
```
The Docker container is built with CUDA Version: 11.0, NVIDIA Driver: 450.57. With the `startx.py` script running inside the docker container and setting `export DISPLAY=:0` I get from `$ glxgears | grep version` that `OpenGL core profile version string: 4.6.0 NVIDIA 450.57` and the like for OpenGL shading language. It does say
```
server glx version string: 1.4
client glx version string: 1.4
GLX version: 1.4
```
for what it's worth.

### Additional information
I'm trying to get headless running — some of the other change I've made are adding `RUN pip install --upgrade pip` to the Dockerfile before `RUN pip3 install -r requirements.txt && ...` as advised by the second FAQ on the [opencv-python pypl page](https://pypi.org/project/opencv-python/) because I was running into the error
```
...
ModuleNotFoundError: No module named 'skbuild'

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-pniw64kt/opencv-python/
The command '/bin/sh -c pip3 install -r requirements.txt && python3 -c "import ai2thor.controller; ai2thor.controller.Controller(download_only=True)"' returned a non-zero code: 1
```
I also added `libgl1-mesa-glx` and `mesa-utils` to the apt-get call, but that was mostly for running the `startx.py` script and `glxinfo`.

Also, there is one `/usr/lib/xorg/Xorg` process already running on this cluster machine — one process, on the four GPUs — which means that my `Xorg` process from `startx.py` is the second extant one — its parent process is gdm-xsession, and its great-grandparent process is gdm3, so I'm pretty sure that has to do with the desktop GUI <-> GPU interface.