Add Dockerfile for building image on arm64

Add Dockerfile.arm64 for building Multus image on arm64.

Actually it's multi-arch capable and works for
amd64 platform too if used.

Signed-off-by: Trevor Tao <trevor.tao@arm.com>

Dockerfile.arm64

@@ -0,0 +1,20 @@
+# This Dockerfile is used to build the image available on DockerHub
+FROM golang:1.13.4 as build
+
+# Add everything
+ADD . /usr/src/multus-cni
+
+#ENV GOARCH "arm64"
+#ENV GOOS "linux"
+
+RUN  cd /usr/src/multus-cni && \
+     ./build
+
+# build arm64 container
+FROM centos:7
+COPY --from=build /usr/src/multus-cni /usr/src/multus-cni
+
+WORKDIR /
+ADD ./images/entrypoint.sh /
+
+ENTRYPOINT ["/entrypoint.sh"]