fix: use unix path separator since path already normalized

In the case for Windows, this line at
frontend/dockerfile/dockerfile2llb/convert.go#L1142
```go
dest += string(filepath.Separator)
```
was adding the `\\` to a path that is already normalized
to unix-format, hence ending up with dest paths like
`/\\` for `C:\\` and `/test\\` for `C:\\test\\`.

the src paths are well normalized too at ~L1290.

fixes moby#4696

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

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1139,7 +1139,10 @@ func dispatchCopy(d *dispatchState, cfg copyConfig) error {
 	}
 
 	if cfg.params.DestPath == "." || cfg.params.DestPath == "" || cfg.params.DestPath[len(cfg.params.DestPath)-1] == filepath.Separator {
-		dest += string(filepath.Separator)
+		// can use unix path separator since
+		// at this point, Windows dest path is already normalized
+		// in `pathRelativeToWorkingDir` above.
+		dest += "/"
 	}
 
 	var copyOpt []llb.CopyOption

solver/llbsolver/file/backend.go

@@ -139,7 +139,7 @@ func docopy(ctx context.Context, src, dest string, action pb.FileActionCopy, u *
 	}
 	destPath, err := cleanPath(action.Dest)
 	if err != nil {
-		return errors.Wrap(err, "cleaning path")
+		return errors.Wrap(err, "cleaning destination path")
 	}
 	if !action.CreateDestPath {
 		p, err := fs.RootPath(dest, filepath.Join("/", action.Dest))