Support override of binary name

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

libnetwork/portmapper/proxy.go

@@ -10,7 +10,7 @@ import (
 	"time"
 )
 
-const userlandProxyCommandName = "docker-proxy"
+var userlandProxyCommandName = "docker-proxy"
 
 type userlandProxy interface {
 	Start() error