container: remove global msg

This frees all container instances of side effects.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/executable_test.go

@@ -9,7 +9,7 @@ import (
 
 func TestExecutable(t *testing.T) {
 	for i := 0; i < 16; i++ {
-		if got := container.MustExecutable(); got != os.Args[0] {
+		if got := container.MustExecutable(container.NewMsg(nil)); got != os.Args[0] {
 			t.Errorf("MustExecutable: %q, want %q",
 				got, os.Args[0])
 		}