Simplify Docker build process and collapse on one file.

[andrewsomething]

This PR aims to simplify Docker build process by collapsing it on one file. To do so, I first needed to allow overriding GOARCH in the Makefile.

A few things to note:

• This incorporates @BobyMCbobs' suggestion in Add multi-stage build, non-root user, multi-arch builds in Dockerfile #855 to run as a non-root user
• This deprecated make native in favor of make build. It was never truly native (just amd64 or 386), and as it now supports being overridden it makes less sense.
• In make docker_build, I copy out the resulting binary with /bin/cp instead of rsync to remove the unneeded dependency.

A few things to verify:

docker build . -t doctl # Build Docker with defaults
docker build --build-arg GOARCH=386 . -t doctl # Build Docker overriding arch

make docker_build # Builds Docker w/ defaults, copies binary to builds/
make docker_build GOARCH=arm64 # Builds Docker overriding arch, `file builds/doctl` shows aarch64 binary

make build # Build binary with defaults
make native # Still works but prints deprecation warning
make build GOARCH=arm64 # Build binary overriding arch

[bentranter]

👍 LGTM

[scotchneat]

👍