Pin regex in docker to 2021.10.8 (GH-2579)

* Pin regex in docker to 2021.10.8
- This is due to 2021.10.8 having arm wheels and newer versions not

I will go see if I can help restore arm build @ https://bitbucket.org/mrabarnett/mrab-regex/issues/399/missing-wheel-for-macosx-and-the-new-m1 soon.

Test: Build on my M1 mac: `docker build -t cooperlees/black .`

* Add in that the pin is only for docker