Build multi-arch Docker images

[skandragon]

I have a Makefile and a Dockerfile that will:

• Build go-clouddriver inside Docker, removing the need to have a specific version installed on the build machine,
• Build for amd64 and arm64 Linux as a single multi-arch image

This is common for us to do for most of our tools here, as we do have some customers attempting to use arm64 for some of the less CPU intensive work.

I'd appreciate suggestions on how to merge them. For now, I've not made a PR because having two Dockerfiles and two Makefiles seems wrong, but build processes are not something to jump into.

https://gist.github.com/skandragon/b5625a885fb5de8fccfa0f6798355e17 for the details.