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.

Dockerfile

@@ -1,10 +1,12 @@
 FROM python:3-slim
 
+# TODO: Remove regex version pin once we get newer arm wheels
 RUN mkdir /src
 COPY . /src/
 RUN pip install --no-cache-dir --upgrade pip setuptools wheel \
     && apt update && apt install -y git \
     && cd /src \
+    && pip install --no-cache-dir regex==2021.10.8 \
     && pip install --no-cache-dir .[colorama,d] \
     && rm -rf /src \
     && apt remove -y git \