Add script and README for publishing pulsar-build image (apache#2222)

*Motivation*

The instructions are missing for publishing `pulsar-build` image.

*Changes*

- Add a script for publishing `pulsar-build` image
- Add a README for the instructions to do so

build/docker/README.md

@@ -29,3 +29,20 @@ docker build -t pulsar-build .
 The image is already available at https://hub.docker.com/r/apachepulsar/pulsar-build
 
 Example: `apachepulsar/pulsar-build:ubuntu-16.04`
+
+## Build and Publish pulsar-build image
+
+> Only committers have permissions on publishing pulsar images to `apachepulsar` docker hub.
+
+### Build pulsar-build image
+
+
+```shell
+docker build -t apachepulsar/pulsar-build:ubuntu-16.04 .
+```
+
+### Publish pulsar-build image
+
+```shell
+publish.sh
+```

build/docker/publish.sh

@@ -0,0 +1,57 @@
+#!/bin/bash
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+
+ROOT_DIR=$(git rev-parse --show-toplevel)
+cd $ROOT_DIR/build/docker
+
+if [ -z "$DOCKER_USER" ]; then
+    echo "Docker user in variable \$DOCKER_USER was not set. Skipping image publishing"
+    exit 1
+fi
+
+if [ -z "$DOCKER_PASSWORD" ]; then
+    echo "Docker password in variable \$DOCKER_PASSWORD was not set. Skipping image publishing"
+    exit 1
+fi
+
+DOCKER_ORG="${DOCKER_ORG:-apachepulsar}"
+
+docker login ${DOCKER_REGISTRY} -u="$DOCKER_USER" -p="$DOCKER_PASSWORD"
+if [ $? -ne 0 ]; then
+    echo "Failed to loging to Docker Hub"
+    exit 1
+fi
+
+if [[ -z ${DOCKER_REGISTRY} ]]; then
+    docker_registry_org=${DOCKER_ORG}
+else
+    docker_registry_org=${DOCKER_REGISTRY}/${DOCKER_ORG}
+    echo "Starting to push images to ${docker_registry_org}..."
+fi
+
+set -x
+
+# Fail if any of the subsequent commands fail
+set -e
+
+# Push all images and tags
+docker push ${docker_registry_org}/pulsar-build:ubuntu-16.04
+
+echo "Finished pushing images to ${docker_registry_org}"