setup automated builds using the release-tools

Use the steps at below link to setup automated builds via
release-tools.

https://github.com/kubernetes-csi/csi-release-tools

- add release-tools via git subtree
- update Makefile to use the release-tools/make
- add cloudbuild links from release-tools
- update Dockerfile that can be used for multi-arch by release tools

Signed-off-by: kmova <kiran.mova@mayadata.io>

Dockerfile

@@ -0,0 +1,22 @@
+# Copyright 2017-2020 The Kubernetes Authors.
+#
+# Licensed 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.
+
+FROM alpine
+LABEL maintainers="Kubernetes Authors"
+LABEL description="NFS subdir external provisioner"
+ARG binary=./bin/nfs-subdir-external-provisioner
+
+RUN apk update --no-cache && apk add ca-certificates
+COPY ${binary} /nfs-subdir-external-provisioner
+ENTRYPOINT ["/nfs-subdir-external-provisioner"]