Configure Dockerfile for integration with Docker Cloud/Dockerhub for (#3850)

automated image creation.

docker/hooks/build

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+# We've placed the Dockerfile under docker/ so that the generically named
+# hooks/ directory doesn't pollute the main directory.  Because we need to
+# COPY the source into the container, we need to use some -f gymnastics to
+# ensure that "COPY . /src" is referring to the repo root, not the directory
+# that contains the Dockerfile.
+# This script gets executed with a pwd of wherever the Dockerfile is.
+docker build --build-arg DOCKER_TAG=${DOCKER_TAG} -t $IMAGE_NAME -f Dockerfile ..