Dockerfile: use GO_VERSION build-arg for overriding Go version

This allows overriding the version of Go without making modifications in the
source code, which can be useful to test against multiple versions.

For example:

    make GO_VERSION=1.13beta1 -f docker.Makefile binary

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 0d3022c6d2)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

dockerfiles/Dockerfile.cross

@@ -1,4 +1,6 @@
-FROM	dockercore/golang-cross:1.12.7
+ARG GO_VERSION=1.12.7
+
+FROM	dockercore/golang-cross:${GO_VERSION}
 ENV     DISABLE_WARN_OUTSIDE_CONTAINER=1
 WORKDIR /go/src/github.com/docker/cli
 COPY    . .