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.

This change removes the block of code and instead
does the "/" appending using the keepSlash logic
that is in system.NormalizePath called in
pathRelativeToWorkingDir() function before.

fixes #4696

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

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1138,10 +1138,6 @@ func dispatchCopy(d *dispatchState, cfg copyConfig) error {
 		return err
 	}
 
-	if cfg.params.DestPath == "." || cfg.params.DestPath == "" || cfg.params.DestPath[len(cfg.params.DestPath)-1] == filepath.Separator {
-		dest += string(filepath.Separator)
-	}
-
 	var copyOpt []llb.CopyOption
 
 	if cfg.chown != "" {
@@ -1562,9 +1558,9 @@ func pathRelativeToWorkingDir(s llb.State, p string, platform ocispecs.Platform)
 	}
 
 	if system.IsAbs(p, platform.OS) {
-		return system.NormalizePath("/", p, platform.OS, false)
+		return system.NormalizePath("/", p, platform.OS, true)
 	}
-	return system.NormalizePath(dir, p, platform.OS, false)
+	return system.NormalizePath(dir, p, platform.OS, true)
 }
 
 func addEnv(env []string, k, v string) []string {

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))