docs(base-images): clarify Ubuntu release example (#25462)

## Description

Clarifies that the Ubuntu 24.04 / `noble` example in the base image
guide is only one example release choice, and that readers should
replace the codename with the Ubuntu release they want to import.

## Related issues or tickets

- Closes #25290

## Reviews

- [x] Technical review
- [ ] Editorial review
- [ ] Product review

Author: dvdksn

content/manuals/build/building/base-images.md

@@ -99,7 +99,9 @@ the distribution you'd like to package as a base image, though that is
 not required for some tools like Debian's [Debootstrap](https://wiki.debian.org/Debootstrap),
 which you can also use to build Ubuntu images.
 
-For example, to create an Ubuntu base image:
+For example, to create an Ubuntu base image using the `noble` codename for
+Ubuntu 24.04, run the following commands. Replace `noble` with the codename
+for the Ubuntu release you want to import:
 
 ```dockerfile
 $ sudo debootstrap noble noble > /dev/null