docs(test-docker-build): add some useful comments

.github/workflows/test-all-packages.yml

@@ -734,19 +734,23 @@ jobs:
       matrix:
         bootstrap-version: ['test', 'main']
     steps:
-      - name: free up additional worker space
+      - name: free up disk space
         run: |
           # Workaround to provide additional free space for testing.
-          #   https://github.com/actions/virtual-environments/issues/2840
+          #   https://github.com/actions/runner-images/issues/2840#issuecomment-790492173
+          # If this turns out not to be enough, maybe look instead at
+          #   https://github.com/actions/runner-images/issues/2840#issuecomment-1540506686
           df -h
           sudo rm -rf /usr/share/dotnet
           sudo rm -rf /opt/ghc
           sudo rm -rf "/usr/local/share/boost"
           sudo rm -rf "$AGENT_TOOLSDIRECTORY"
+          echo "=== After cleanup:"
           df -h
-
       - uses: actions/checkout@v3
       - name: docker build (sdk)
+        # Produces ghcr.io/agoric/agoric-sdk:latest used in the following upgrade test.
+        # TODO: Build this only once, not for every bootstrap-version.
         run: cd packages/deployment && ./scripts/test-docker-build.sh | $TEST_COLLECT
       - name: docker build upgrade test
         run: |