Multi-arch Docker images for distribution

Build to create images with binary

Update the build to create images of the binaries for each
of the supported platforms: darwin, armhf, arm64, windows,
and x86_64 (linux)

Also updated the travis file in order to push these images
up to docker hub and create a manifest file for easy
multi-arch image pulls

Signed-off-by: Burton Rheutan <[email protected]>