Add build-base to build stage of docker image

ruamel.yaml depends on ruamel.yaml.clib, which requires
compilation on alpine. It did not seem to require the
packages in build-base before, but it does now.

Dockerfile

@@ -1,7 +1,7 @@
 ARG ALPINE_VERSION=3.12.0
 FROM alpine:${ALPINE_VERSION}
 
-RUN apk add --no-cache git python3 python3-dev py-pip
+RUN apk add --no-cache git python3 python3-dev py-pip build-base
 
 # build wheels in first image
 ADD . /tmp/src