Runner: reachable actions cache on docker0:8088, allow the workspace bind in valid_volumes

Co-Authored-By: Claude Fable 5.1 <[email protected]>

deploy/runner/README.md

@@ -18,6 +18,10 @@ the `spencerm` user through the Docker socket. No sudo is needed.
   "bind source path does not exist". The runner removes what the job started when the job ends; the
   workflow also runs `down -v` itself.
 - Capacity 2: server and frontend run at the same time; e2e waits for both.
+- The actions cache (`setup-python` pip cache, `setup-node` npm cache) is served by the runner on
+  docker-1's docker0 address, port 8088, published in compose.yaml. Job containers cannot reach the
+  runner's own network; before this was set, every cache restore and save timed out after about
+  four and a half minutes, which made the server job take 13 minutes instead of 4.
 
 ## Install (once)