Add missing library (libffi-dev) during wheel builds

Author: ldeluigi

Dockerfile

@@ -10,9 +10,9 @@ RUN chmod +x /usr/local/bin/plantuml
 
 # Download Python Markdown + dependencies
 COPY config/requirements.txt /usr/local/src/requirements.txt
-RUN apk add --no-cache gcc git musl-dev && \
+RUN apk add --no-cache gcc git musl-dev libffi-dev && \
   pip install --no-cache-dir -r /usr/local/src/requirements.txt && \
-  apk del gcc musl-dev
+  apk del gcc musl-dev libffi-dev
 
 # Configuration
 COPY config/mkdocs.yml /usr/local/src/mkdocs.yml