BUG: Clipboard tests failing in Dockerfile
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---


#### Code Sample, a copy-pastable example

```python
pytest pandas/tests/io/test_clipboard.py 
```

#### Problem description
Last three clipboard tests are failing in a Docker image running on Mac. Dug into the pyperclip code and the issue appears to be that the Docker container does not have a DISPLAY environment variable set correctly. Pyperclip code does not recognize my machine as a Mac, so it tries to proceed as if its a Linux.

I've tried manually setting it, and the tests run through completion but the assertions fail and I get `Could not open display`. Does anyone have an idea how to fix this? Or is Dockerfile not supported for Macs? 