Update outdated url to ubuntu source code (#22626)

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## Description

I set a new URL for the source code because the old ones refer to the
commit of the file, which is not used now, because Ubuntu for Docker is
built from livecd scripts.

## Related issues or tickets

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## Reviews

<!-- Notes for reviewers here -->
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- [ ] Technical review
- [x] Editorial review
- [ ] Product review

---------

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>
Co-authored-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>
Co-authored-by: Tianon Gravi <admwiggin@gmail.com>

Author: 3 people

content/manuals/build/building/best-practices.md

@@ -442,7 +442,7 @@ reduces the image size, since the apt cache isn't stored in a layer. Since the
 `RUN` statement starts with `apt-get update`, the package cache is always
 refreshed prior to `apt-get install`.
 
-Official Debian and Ubuntu images [automatically run `apt-get clean`](https://github.com/moby/moby/blob/03e2923e42446dbb830c654d0eec323a0b4ef02a/contrib/mkimage/debootstrap#L82-L105), so explicit invocation is not required.
+Official Debian and Ubuntu images [automatically run `apt-get clean`](https://github.com/debuerreotype/debuerreotype/blob/c9542ab785e72696eb2908a6dbc9220abbabef39/scripts/debuerreotype-minimizing-config#L87-L109), so explicit invocation is not required.
 
 #### Using pipes