Add Dockerfile for riscv64 architectures for helloworld example

[maiquanghiep]

Related-issue: https://github.com/open-horizon/anax/issues/2373 To port an example to riscv64 there are 2 items that I need to do:

• Create Dockerfile
• Add "Riscv64 ARCH" option to Makefile. In this PR, I only create Dockerfile because some of the Makefile commands couldn't be tested (E.g make test, make publish) because currently the Anax is not supported in Riscv64. So I guess it's better to have another PR for that later on. About the base image, most of the examples use the alpine but there's no alpine official image yet. My suggestion is to use the busybox base image mentioned in RISCVSUMMIT (https://riscv.org/wp-content/uploads/2019/12/12.10-14.50c-The-RISC-V-Journey-Through-Containers-to-the-Cloud.pdf)

[joewxboy]

Examples adds support for RISC-V

[dabooz]

@maiquanghiep Please rebase this PR on the latest master so that we can merge the commit. Thanks.

[joewxboy]

@t-fine can/should this be rebased and merged?