[Bug Report] docker/container.py enter fails if `DISPLAY` env is not defined

[GiulioRomualdi]

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Describe the bug

I'm trying to enter the container built into a server without display. The enter command fails
as shown above

Steps to reproduce

I started the container as written in the documentation python3 docker/container.py start

Then when I entered the container I got the following error:

python3 docker/container.py enter
[INFO] Using container profile: base
[INFO] X11 Forwarding is disabled from the settings in '.container.cfg'
[INFO] X11 forwarding is disabled. No action taken.
[INFO] Entering the existing 'isaac-lab-base' container in a bash session...

Traceback (most recent call last):
  File "/home/iit.local/gromualdi/robot-code/IsaacLab/docker/container.py", line 127, in <module>
    main(args_cli)
  File "/home/iit.local/gromualdi/robot-code/IsaacLab/docker/container.py", line 111, in main
    ci.enter()
  File "/home/iit.local/gromualdi/robot-code/IsaacLab/docker/utils/container_interface.py", line 146, in enter
    f"DISPLAY={os.environ['DISPLAY']}",
  File "/usr/lib/python3.10/os.py", line 680, in __getitem__
    raise KeyError(key) from None
KeyError: 'DISPLAY

System Info

Describe the characteristics of your environment:

• Commit: e00d625 (v1.2.0)
• Isaac Sim Version: 4.2.0-rc.18+release.16044.3b2ed111.gl
• OS: Ubuntu 22.04
• GPU: NVIDIA A100 80GB PCIe
• CUDA: 12.2
• GPU Driver: 535.183.01

Additional context

Add any other context about the problem here.

Checklist

• I have checked that there is no similar issue in the repo (required)
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo

Acceptance Criteria

Add the criteria for which this task is considered done. If not known at issue creation time, you can add this once the issue is assigned.

• Make the script running even if the display is not defined

[GiulioRomualdi]

#1163 attempts to solve the issue

[Dhoeller19]

Thanks a lot @GiulioRomualdi for the report and the fix!
We'll get this merged asap.