fix: normalizePath function to standardize the paths correctly

We have an edge case situation coming from a path like C:\\
The drive letter is stipped out and \\ remains and therefore
appended with / to get `/\\`.

normalizePath is suppposed to also standardize this path to
unix format and it wasn't doing so, the path was returning as
is. With this minor fix, `/` will be returned instead.

fixes moby#4696

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

client/llb/fileop.go

@@ -5,6 +5,7 @@ import (
 	_ "crypto/sha256" // for opencontainers/go-digest
 	"os"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -801,6 +802,8 @@ func (f *FileOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 
 func normalizePath(parent, p string, keepSlash bool) string {
 	origPath := p
+	// first, normalize everything to unix path
+	p = filepath.ToSlash(p)
 	p = path.Clean(p)
 	if !path.IsAbs(p) {
 		p = path.Join("/", parent, p)

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))