Unexpectedly the team which runs the Go docker images have removed the
arm/v6 image which means that the rclone docker images no longer
build.
One of the recommended fixes is what we've done here - switch to the
alpine builder. This has the advantage that it actually builds arm/v6
architecture unlike the previous builder which build arm/v5.
See: https://github.com/docker-library/golang/issues/502
GOOS and GOARCH being set like this makes it impossible to compile on
other archs.
For me GOARCH prevents compilation on my ARM machine.
For others GOOS will prevent windows.
xref https://github.com/rclone/rclone/issues/4086
* Use a multi-stage build to reduce final image size.
* Run 'quicktest' make target before building.
* Built binary won't run on Alpine unless statically linked.
