install build-essential to compile packages and use multi-stage build

- Install build-essential to avoid build issues like psf#2568 when dependencies don't have prebuilt wheels available
- Use multi-stage build instead of trying to purge packages and cache from the image
  Copying `/root/.local/` installs only black's built Python dependencies (< 20 MB).
  So the image is barely larger than python:3-slim base image

Dockerfile

@@ -1,16 +1,17 @@
-FROM python:3-slim
+FROM python:3-slim AS builder
 
-# 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 \
+    # Install build tools to compile dependencies that don't have prebuilt wheels
+    && apt update && apt install -y git build-essential \
     && 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 \
-    && apt autoremove -y \
-    && rm -rf /var/lib/apt/lists/*
+    && pip install --user --no-cache-dir .[colorama,d]
+
+FROM python:3-slim
+
+# copy only Python packages to limit the image size
+COPY --from=builder /root/.local /root/.local
+ENV PATH=/root/.local/bin:$PATH
 
 CMD ["black"]