Log docker system info in CI (#19480)

Follow-up to
https://github.com/element-hq/synapse/pull/19460#discussion_r2819139638
and https://github.com/element-hq/synapse/pull/19475

.github/workflows/tests.yml

@@ -693,6 +693,12 @@ jobs:
         with:
           path: synapse
 
+      # Log Docker system info for debugging (compare with your local environment) and
+      # tracking GitHub runner changes over time (can easily compare a run from last
+      # week with the current one in question).
+      - run: docker system info
+        shell: bash
+
       - name: Install Rust
         uses: dtolnay/rust-toolchain@e97e2d8cc328f1b50210efc529dca0028893a2d9 # master
         with:

changelog.d/19480.misc

@@ -0,0 +1 @@
+Log `docker system info` in CI so we have a plain record of how GitHub runners evolve over time.