dap: file explorer uses the wrong path when looking up mounts

It accidentally retrieved the relative path to the mount directory
instead of the full path which resulted in always returning the root
directory.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

dap/variables.go

@@ -447,7 +447,7 @@ func lookupPath(path string, mounts map[string]gateway.Reference) (remainder str
 	for p, r := range mounts {
 		if len(p) > len(prefix) && strings.HasPrefix(path, p) {
 			prefix = p
-			remainder, _ = filepath.Rel(prefix, p)
+			remainder, _ = filepath.Rel(prefix, path)
 			ref = r
 		}
 	}