Fixes running the docker container when the DISPLAY env variable is not defined (#1163)

# Description

This PR attempts to fix #1162 by adding the possibility to do not pass
the `DISPLAY` env to the docker exec command if the variable is not
defined

Fixes #1162

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

Author: GiulioRomualdi

CONTRIBUTORS.md

@@ -42,6 +42,7 @@ Guidelines for modifications:
 * Chenyu Yang
 * David Yang
 * Gary Lvov
+* Giulio Romualdi
 * HoJin Jeon
 * Jean Tampon
 * Jia Lin Yuan

docker/utils/container_interface.py

@@ -142,8 +142,7 @@ def enter(self):
                 "exec",
                 "--interactive",
                 "--tty",
-                "-e",
-                f"DISPLAY={os.environ['DISPLAY']}",
+                *(["-e", f"DISPLAY={os.environ['DISPLAY']}"] if "DISPLAY" in os.environ else []),
                 f"{self.container_name}",
                 "bash",
             ])