This moves all the release builds into a Dockerfile which is a bit
cleaner for setting up our build environment.
Non-linux/amd64 builds are cross-compiled.
Currently onlinux linux/amd64, linux/arm64, and windows/amd64 are
supported, but is easy to add more, provided their is a cross-compile
toolchain available for it.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
