Dockerfile.test: add GOLANG_IMAGE build arg to allow overriding
This allows the base image itself to be overridden with an alternative image.
# docker build -t containerd-test --build-arg RUNC_VERSION=v1.0.0-rc94 -f Dockerfile.test ../
ARG GOLANG_VERSION=1.16.6
+ARG GOLANG_IMAGE=golang
-FROM golang:${GOLANG_VERSION} AS golang
+FROM ${GOLANG_IMAGE}:${GOLANG_VERSION} AS golang
# Install runc
FROM golang AS runc
