The solution was easy enough. Add these 2 lines to your RUN command:
rm -rf /var/cache/apk && \
mkdir /var/cache/apk && \
In Dockerfile ...
RUN apk add --update --no-cache bash \
git \
make \
clang \
g++ \
go && \
mkdir -p $REPO && \
mkdir -p $GODIR/src && \
rm -rf /usr/share/man && \
rm -rf /var/cache/apk && \
mkdir /var/cache/apk && \
apk del git clang