Add dockerhub build hook

This should fix #5049

In order to build the proper raiden version from `Dockerfile`, we need
to pass the intended `RAIDENVERSION` build argument. According to
docker/hub-feedback#508 the current way of
doing this is via build hook.
dockerhub has the `SOURCE_BRANCH` environment variable available, that
should translate just fine to the values we want in `RAIDENVERSION`.

docker/hooks/build

@@ -0,0 +1,2 @@
+#!/bin/sh
+docker build --build-arg RAIDENVERSION=${SOURCE_BRANCH} -t $IMAGE_NAME .