Parametrize dockerBuildImageName

[bwplotka]

It would be nice to parameterize promu to use different golang builder image, plus make you we push golang builder to dockerhub as well here:

promu/cmd/crossbuild.go

Line 32 in d629dfc

dockerBuilderImageName = "quay.io/prometheus/golang-builder"

Rationales:
quay.io is down for ~day and our CI is using promu crossbuild, it would be nice to be able to quickly switch to dockerhub in those cases. (: Otherwise we are kind of stuck: thanos-io/thanos#2631