drb-update: prune BuildKit cache and stale images after every update (#41)

node-002's root disk hit 100% (39.5 GB build cache + 16 GB unused images)
and the node silently stopped recording for 17 days: BUILD=1 runs
`docker compose build` on every update and nothing ever pruned the cache;
the existing prunes only drop dangling images and old src- pins.

After `up -d`: `builder prune -af --filter until=72h` and
`image prune -af --filter until=168h`. Neither touches anything a running
container uses; the age filters keep the next build's cache warm.

Disk-free reporting to the portal (the other half of #41) is not in this
change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-edge-node/tests/test_host_updater.py

@@ -151,6 +151,17 @@ def test_mqtt_sentinel_triggers_update_and_is_consumed(env):
     assert "evil" not in calls and "rm -rf" not in calls
 
 
+def test_update_prunes_build_cache_and_stale_images_after_up(env):
+    # node-26#41: the build cache was never pruned and filled node-002's disk.
+    (env["stub"] / "response").write_text(json.dumps({"update_requested_at": "2026-10-04T12:00:00+00:00"}))
+    assert _run(env).returncode == 0
+    lines = _calls(env).splitlines()
+    up = next(i for i, ln in enumerate(lines) if ln.startswith("docker compose up -d"))
+    builder = next(i for i, ln in enumerate(lines) if ln.startswith("docker builder prune -af --filter until="))
+    images = next(i for i, ln in enumerate(lines) if ln.startswith("docker image prune -af --filter until="))
+    assert up < builder and up < images  # only after the new containers are running
+
+
 def test_c2_response_is_never_used_as_a_ref_or_command(env):
     (env["stub"] / "response").write_text(json.dumps({"update_requested_at": "$(touch /tmp/pwned); main"}))
     r = _run(env)

scripts/drb-update.sh

@@ -225,6 +225,14 @@ do_update() {
         done || true
   fi
   docker image prune -f >/dev/null 2>&1 || true
+
+  # node-26#41: neither of the above ever freed BuildKit cache or untagged-
+  # but-unused images, so a BUILD=1 node grew ~40 GB of build cache plus
+  # 16 GB of stale images until the root disk hit 100% and recording silently
+  # stopped (node-002, 17 days). Both prunes skip anything a container uses;
+  # the age filters keep the cache warm for the next build.
+  docker builder prune -af --filter until=72h >/dev/null 2>&1 || true
+  docker image prune -af --filter until=168h >/dev/null 2>&1 || true
 }
 
 main "$@"