Add DOCKER_EXPERIMENTAL environment variable

The DOCKER_EXPERIMENTAL environment variable drives the activation of
the 'experimental' build tag.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Upstream-commit: ca6722f1c5e4e2017a71e33579d91ac0d9ea2a25
Component: engine

components/engine/docker/docker.go

@@ -16,6 +16,7 @@ import (
 	flag "github.com/docker/docker/pkg/mflag"
 	"github.com/docker/docker/pkg/reexec"
 	"github.com/docker/docker/pkg/term"
+	"github.com/docker/docker/utils"
 )
 
 const (
@@ -59,6 +60,10 @@ func main() {
 		setLogLevel(logrus.DebugLevel)
 	}
 
+	if utils.ExperimentalBuild() {
+		logrus.Warn("Running experimental build")
+	}
+
 	if len(flHosts) == 0 {
 		defaultHost := os.Getenv("DOCKER_HOST")
 		if defaultHost == "" || *flDaemon {