Dockerfile for Alpine image with cached packages

To speed up builds:

# syntax=docker/dockerfile:1.5
FROM alpine
RUN --mount=type=cache,target=/var/cache/apk <<INSTALL
   ln -s /var/cache/apk /etc/apk/cache
   apk add -U \
       curl \
       bash \
       go \
       clang
   rm /etc/apk/cache
INSTALL