Add support for autodetected HOME from USER (if HOME is unset)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

integration-cli/docker_cli_build_test.go

@@ -700,7 +700,7 @@ func TestBuildRelativeWorkdir(t *testing.T) {
 
 func TestBuildEnv(t *testing.T) {
 	name := "testbuildenv"
-	expected := "[HOME=/ PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin PORT=2375]"
+	expected := "[PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin PORT=2375]"
 	defer deleteImages(name)
 	_, err := buildImage(name,
 		`FROM busybox

integration-cli/docker_cli_run_test.go

@@ -676,13 +676,13 @@ func TestEnvironment(t *testing.T) {
 
 	goodEnv := []string{
 		"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
-		"HOME=/",
 		"HOSTNAME=testing",
 		"FALSE=true",
 		"TRUE=false",
 		"TRICKY=tri",
 		"cky",
 		"",
+		"HOME=/root",
 	}
 	sort.Strings(goodEnv)
 	if len(goodEnv) != len(actualEnv) {