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>

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 "$@"