reexec: Use in-memory binary on linux instead of os.Args[0]

This keeps reexec working properly even if the on-disk binary was replaced.

Signed-off-by: Tibor Vass <tibor@docker.com>

reexec/command_windows.go

@@ -6,6 +6,12 @@ import (
 	"os/exec"
 )
 
+// Self returns the path to the current process's binary.
+// Uses os.Args[0].
+func Self() string {
+	return naiveSelf()
+}
+
 // Command returns *exec.Cmd which have Path as current binary.
 // For example if current binary is "docker.exe" at "C:\", then cmd.Path will
 // be set to "C:\docker.exe".