GHA: drop 3rd-party apt source docker.list

Seen on `ubuntu-slim` runners.

Also:
- extend to two jobs to `ls -l` package sources on both ubuntu-latest,
  ubuntu-24.04-arm, and ubuntu-slim.
- drop former stray apt source `ondrej-ubuntu-php-noble.sources`.
  It's no longer present on the image.

Closes #21344

.github/workflows/checkdocs.yml

@@ -100,7 +100,7 @@ jobs:
 
       - name: 'install'
         run: |
-          sudo rm -f /etc/apt/sources.list.d/{azure-cli.sources,microsoft-prod.list,ondrej-ubuntu-php-noble.sources}
+          sudo rm -f /etc/apt/sources.list.d/{azure-cli.sources,docker.list,microsoft-prod.list}
           sudo apt-get -o Dpkg::Use-Pty=0 update
           sudo apt-get -o Dpkg::Use-Pty=0 install aspell aspell-en
           python3 -m venv ~/venv