Document CI runner Docker pruning setup in deploy/README.md

Gitea Actions runners accumulate buildkit cache, stale containers,
and unused images. Without periodic cleanup the disk fills and builds
stall during image push (observed: build-omos hung at 'pushing layers'
for 1.5h on a 77%-full disk).

Add a 'CI runner maintenance' section to deploy/README.md with two
cleanup layers: a daily cron job (prunes anything >72h old) and
Docker daemon builder GC (caps buildkit cache at 10 GB).

deploy/README.md

@@ -238,6 +238,42 @@ This means:
 - To restore the baked defaults any time: `cp /etc/skel-devbox/.bash_aliases ~/` (or delete the file and recreate the container).
 - To diff your current config against what the image ships: `diff ~/.bash_aliases /etc/skel-devbox/.bash_aliases`.
 
+### CI runner maintenance: automatic Docker pruning
+
+Gitea Actions runners accumulate Docker build cache, stale buildkit containers, and unused images over time. Without periodic cleanup, the runner's disk fills up and builds stall during the image-push phase (symptom: `#61 exporting to image` / `pushing layers` hangs indefinitely while buildkit repeatedly re-authenticates with Docker Hub).
+
+Set up two layers of automatic cleanup on the runner host:
+
+**1. Daily cron job** — prunes images, containers, and build cache older than 72 hours:
+
+```bash
+sudo tee /etc/cron.daily/docker-prune <<'EOF'
+#!/bin/sh
+docker system prune -af --filter "until=72h" > /var/log/docker-prune.log 2>&1
+docker builder prune -af --filter "until=72h" >> /var/log/docker-prune.log 2>&1
+EOF
+sudo chmod +x /etc/cron.daily/docker-prune
+```
+
+**2. Docker daemon builder GC** — caps buildkit cache at 10 GB (Docker 23.0+):
+
+Add to `/etc/docker/daemon.json` (create if absent):
+
+```json
+{
+  "builder": {
+    "gc": {
+      "enabled": true,
+      "defaultKeepStorage": "10GB"
+    }
+  }
+}
+```
+
+Then `sudo systemctl restart docker`.
+
+Both are safe to run on a machine that also hosts long-running containers (like opencode-devbox) — `docker system prune` only removes *unused* images and *stopped* containers, never running ones.
+
 ### Troubleshooting: SSH hangs or "banner exchange" timeouts
 
 If SSH to the VM intermittently fails with `Connection timed out during banner exchange` or pure TCP connect timeouts — especially after the first few successful connects in a short window — the cause is almost certainly your ISP's CGNAT (Carrier-Grade NAT), not the VM.