Add Docker Hub build hooks

hooks/README.md

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+# Docker Hub Automated Build Hooks
+
+This directory contains Docker Hub [Automated Build](https://docs.docker.com/docker-hub/builds/advanced/) hooks.
+This is needed since we publish multiple images as part of a single build:
+* argoproj/workflow-controller:latest
+* argoproj/argoexec:latest
+* argoproj/argocli:latest
+
+It relies the DOCKER_REPO and DOCKER_TAG environment variables that are set by Docker Hub during
+the build.
+
+Hooks can be tested using:
+```
+DOCKER_REPO=index.docker.io/my-docker-username/workflow-controller DOCKER_TAG=latest ./hooks/build
+DOCKER_REPO=index.docker.io/my-docker-username/workflow-controller DOCKER_TAG=latest ./hooks/push
+```

hooks/build

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+#!/bin/bash -e
+docker_org=$(echo $DOCKER_REPO | rev | cut -d / -f 2- | rev)
+targets="workflow-controller argoexec argocli"
+for target in $targets; do
+    image_name="${docker_org}/${target}:${DOCKER_TAG}"
+    docker build --target $target -t $image_name .
+done

hooks/push

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+#!/bin/bash -e
+docker_org=$(echo $DOCKER_REPO | rev | cut -d / -f 2- | rev)
+targets="workflow-controller argoexec argocli"
+for target in $targets; do
+    image_name="${docker_org}/${target}:${DOCKER_TAG}"
+    docker push $image_name
+done