ci: Change image-warming pull to happen in parallel

.github/actions/warm-test-images/action.yml

@@ -50,8 +50,35 @@ runs:
           echo "::warning::could not warm $canonical from GHCR mirror; falling back to Docker Hub"
         }
 
-        warm matrixdotorg/synapse:v1.126.0 synapse:v1.126.0
-        warm rnwood/smtp4dev:v3.1          smtp4dev:v3.1
-        warm postgres:16.3                 postgres:16.3
-        warm ghcr.io/navikt/mock-oauth2-server:4.0.1 mock-oauth2-server:4.0.1
-        warm caddy:2.10.2-alpine           caddy:2.10.2-alpine
+        # Pull the five in parallel. They are distinct images, so the daemon
+        # fetches them concurrently and the step costs about as much as the
+        # slowest one instead of the sum; every shard pays this, so the
+        # difference is machine-hours across a run. Each pull's output goes to
+        # its own file and is replayed in a fixed order afterwards, because
+        # interleaved `docker pull` progress from five images is unreadable
+        # exactly when someone is trying to work out which mirror failed.
+        warm_bg() {
+          log="$(mktemp)"
+          logs+=("$log")
+          warm "$1" "$2" >"$log" 2>&1 &
+          pids+=($!)
+        }
+
+        logs=()
+        pids=()
+        warm_bg matrixdotorg/synapse:v1.126.0 synapse:v1.126.0
+        warm_bg rnwood/smtp4dev:v3.1          smtp4dev:v3.1
+        warm_bg postgres:16.3                 postgres:16.3
+        warm_bg ghcr.io/navikt/mock-oauth2-server:4.0.1 mock-oauth2-server:4.0.1
+        warm_bg caddy:2.10.2-alpine           caddy:2.10.2-alpine
+
+        # `warm` never fails (it downgrades a miss to a warning), so nothing
+        # here needs to inspect exit status — but wait on each pid rather than
+        # bare `wait` so a future warm that does fail can be noticed.
+        for pid in "${pids[@]}"; do
+          wait "$pid" || true
+        done
+        for log in "${logs[@]}"; do
+          cat "$log"
+          rm -f "$log"
+        done