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.
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
