container: use /proc/self/exe directly

This is a more reliable form of pathname to self and also cheaper than os.Executable.

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

container/executable.go

@@ -28,6 +28,9 @@ func copyExecutable(msg message.Msg) {
 	}
 }
 
+// MustExecutable calls [os.Executable] and terminates the process on error.
+//
+// Deprecated: This is no longer used and will be removed in 0.4.
 func MustExecutable(msg message.Msg) string {
 	executableOnce.Do(func() { copyExecutable(msg) })
 	return executable