chore: Prune Ubuntu environment before building docker images (#3571)

Docker builds are suddenly failing for a lack of disk space. Trying out
a suggestion in https://github.com/orgs/community/discussions/179725.

Author: effigies

.github/workflows/docker.yml

@@ -41,6 +41,11 @@ jobs:
       contents: read
       packages: write
     steps:
+      - name: Prune container
+        run: |
+          sudo rm -rf /usr/share/dotnet /usr/local/lib/android /opt/hostedtoolcache
+          docker system prune -af
+
       - name: Checkout repository
         uses: actions/checkout@v5
         with: