.github: workflows: build: add cleanup job before build

Add a `cleanup` job that runs before `build` to prune stale Docker
resources and old `/tmp` entries on the self-hosted runner. This
prevents disk space issues from accumulating CI leftovers.

Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>

Author: simensrostad

.github/workflows/build.yml

@@ -14,7 +14,22 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  cleanup:
+    runs-on: self-hosted
+    steps:
+      - name: Prune old Docker resources
+        run: docker system prune --force --filter "until=48h"
+
+      - name: Remove stale runner temp entries
+        run: |
+          # GitHub Actions stores per-job temp files under _work/_temp.
+          runner_temp_root="$(dirname "${RUNNER_TEMP}")"
+          find "$runner_temp_root" -mindepth 1 -maxdepth 1 -mmin +2880 \
+            ! -path "$RUNNER_TEMP" \
+            -exec rm -rf {} + 2>/dev/null || true
+
   build:
+    needs: cleanup
     runs-on: self-hosted
     container: ghcr.io/zephyrproject-rtos/ci:v0.29.2
     env: