Add promtool to docker build.
Add promtool to docker build.

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,kangwoo/prometheus,prometheus/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,amorken/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,amorken/prometheus,kangwoo/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,amorken/prometheus,kangwoo/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,amorken/prometheus,Gouthamve/prometheus