Convert black back to a single stage based on review comments

black/Dockerfile

@@ -1,18 +1,13 @@
-FROM python:3-alpine as black-build
-
-# we don't want gcc in the final image, so let's do a multi stage build
-RUN apk update && apk add --no-cache build-base
-RUN pip install --user --upgrade black
-
 FROM python:3-alpine
 
 LABEL io.whalebrew.name 'black'
 LABEL io.whalebrew.config.working_dir '/workdir'
 WORKDIR /workdir
 
-# copy only the installed files from black
-COPY --from=black-build /root/.local /root/.local
-ENV PATH=/root/.local/bin:$PATH
+# we don't want gcc in the final image, so remove it in one docker layer
+RUN apk add --no-cache build-base \
+    && pip install --no-cache-dir --upgrade black \
+    && apk del build-base
 
 ENTRYPOINT ["black"]
 CMD ["--help"]