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>

api/client/info.go

@@ -87,6 +87,7 @@ func (cli *DockerCli) CmdInfo(args ...string) error {
 			fmt.Fprintf(cli.out, " %s\n", attribute)
 		}
 	}
+	fmt.Fprintf(cli.out, "Experimental: %t\n", info.ExperimentalBuild)
 
 	return nil
 }