proxy: simply the Dockerfile by shuffling code under vendor/

Note there is a slight name clash between the final binary 'proxy'
and the library formerly known as 'proxy'. Resolve this by calling
the library 'libproxy'.

Signed-off-by: David Scott <dave.scott@docker.com>

alpine/packages/proxy/main.go

@@ -6,7 +6,7 @@ import (
 	"log"
 	"net"
 	"os"
-	"pkg/proxy"
+	"libproxy"
 	"strings"
 )
 
@@ -17,7 +17,7 @@ func main() {
 	if err != nil {
 		sendError(err)
 	}
-	p, err := proxy.NewProxy(host, container)
+	p, err := libproxy.NewProxy(host, container)
 	if err != nil {
 		unexposePort(host)
 		sendError(err)