Add news entry for Ubuntu Miniforge base image (#2506)

Author: hmaarrfk

news/2025-05-16-new-ubuntu-base-for-miniforge-docker-images.md

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+# Updating our Ubuntu base for Miniforge Docker images (20.04 → 24.04)
+
+The base image for our Ubuntu Dockerfiles has been upgraded from Ubuntu 20.04
+(focal) to Ubuntu 24.04 (noble) in
+[PR #145](https://github.com/conda-forge/miniforge-images/pull/145).
+This change ensures continued support and access to newer packages and system
+libraries.
+
+Downstream users building on top of the Ubuntu variant of our containers should
+verify compatibility with the updated environment.
+
+Thanks to @rpanai for the contribution and to the reviewers for their input.