Installs build tools for installing pyyaml (#15)

When attempting to install pyyaml, pip fails to find the `gcc` executable.

Author: mike-carey

Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3-alpine
 
-RUN pip install 'yamllint>=1.25.0' && \
-    apk add --no-cache bash && \
+RUN apk add --no-cache bash gcc musl-dev && \
+    pip install 'yamllint>=1.25.0' && \
     rm -rf ~/.cache/pip
 
 ADD entrypoint.sh /entrypoint.sh