fix: dot path normalized correctly for COPY

In my previous fix on moby#4825, I had removed this line
knowing that all that had been addressed in `pahtRelativeToWorkingDir`:

```go
	if cfg.params.DestPath == "." // <-- this one
		|| cfg.params.DestPath == "" ||
		cfg.params.DestPath[len(cfg.params.DestPath)-1] == filepath.Separator {
		dest += string(filepath.Separator)
	}
```

However, I had overlooked the `"."` scenario. `""`, `"/"` are all handled
correctly in `system.NormalizePath()`.

This change therefore undoes this, to make sure "." is transformed
correctly to "./" during COPY operation.

fixes moby#5070

Signed-off-by: Anthony Nandaa <[email protected]>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1319,6 +1319,12 @@ func dispatchCopy(d *dispatchState, cfg copyConfig) error {
 		return err
 	}
 
+	// handle case for "." path
+	// using "/" since paths already nomalized to UNIX style
+	if cfg.params.DestPath == "." {
+		dest += "/"
+	}
+
 	var copyOpt []llb.CopyOption
 
 	if cfg.chown != "" {