Finally working commit; some dependencies must have been updated.

- I didn't change anything and it broke
- guessing some packages used to be included in an image that
- are no longer there.
- explicitly adding another apk call to include dependencies
- fixed the issue.

docker/dockerfile

@@ -5,12 +5,16 @@ RUN apk update
 
 RUN apk add \
     python3 \ 
-    python-dev \
+    python3-dev \
     libffi-dev
 
-RUN python3 -m pip install --upgrade pip
+RUN apk add --no-cache curl python pkgconfig python-dev openssl-dev libffi-dev musl-dev make gcc
 
-RUN python3 -m pip install slackclient==1.0.0
+RUN pip3 install --upgrade pip
+
+RUN pip3 install --upgrade setuptools
+
+RUN pip3 install slackclient
 
 COPY ./ ./