build/build-release.sh: parametrize golang Docker image
build/build-release.sh: parametrize golang Docker image

So different version of Go can be tested on demand and the version can
be easily updated.

Signed-off-by: Mateusz Gozdek <c74780982fca069d559e5c78b4677476dc80851c@kinvolk.io>

kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube