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/container.go

@@ -525,7 +525,7 @@ func New(ctx context.Context, msg message.Msg) *Container {
 	p := &Container{ctx: ctx, msg: msg, Params: Params{Ops: new(Ops)}}
 	c, cancel := context.WithCancel(ctx)
 	p.cancel = cancel
-	p.cmd = exec.CommandContext(c, MustExecutable(msg))
+	p.cmd = exec.CommandContext(c, fhs.ProcSelfExe)
 	return p
 }