Minor change to the Dockerfile to improve build speed (#1139)

"go mod download" does not depend on the VERSION env var, so moving the ARG directive after the RUN will allow better use of the Docker build cache - subsequent builds on the same machine need only re-run the "go mod download" if go.mod or go.sum has changed, rather than re-running it any time the VERSION value passed from the Makefile has changed

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>