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 shell
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c6281bc4383b7f9eab617fd73601e8594c93365b)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Conflicts:
* Dockerfile, Dockerfile.e2e, Dockerfile.simple, Dockerfile.windows:
(due to Go version difference, missing CROSS etc.)