Update dockerfile path in contributing docs (#13751)
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## Summary
Sets the correct path to the builder dockerfile in the contributing
docs.
---------
Co-authored-by: konsti <konstin@mailbox.org>
