go: use COPY instead of ADD in Dockerfile
go: use COPY instead of ADD in Dockerfile

COPY is less powerful than ADD so prefer COPY in this case.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit