CI: cache Android SDK/NDK, Gradle, and npm across APK builds

Persist three Docker volumes into the build job (android-sdk, gradle home, npm
cache) and enable the Gradle build cache, so repeat builds skip the ~2GB NDK
download, Maven dependency resolution, and unchanged native/Kotlin compilation
— cutting builds from ~1h to a few minutes after the first run.

The runner must whitelist these volumes via config.yaml (container.valid_volumes);
deploy-runner.sh now writes that config, pre-creates the volumes, and starts the
daemon with --config. Requires re-running deploy-runner.sh on the runner host
before the next tag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

runner/README.md

@@ -30,6 +30,27 @@ The runner advertises the `docker` label; the build workflow (`.forgejo/workflow
 2. Push a tag: `git tag v0.1.0 && git push origin v0.1.0`.
 3. The runner builds `camp-scan-v0.1.0.apk` and attaches it to a Forgejo release; Obtainium picks it up.
 
+## Build caching (fast repeat builds)
+
+The first APK build takes ~1h (downloads Gradle + the ~2GB Android NDK, resolves
+all Maven deps, compiles native modules). After that it should drop to a few
+minutes because three persistent Docker volumes are reused across runs:
+
+| Volume | Holds |
+|---|---|
+| `camptickets-android-sdk` → `/opt/android-sdk` | SDK, NDK, CMake |
+| `camptickets-gradle` → `/root/.gradle` | Gradle dist, Maven deps, local build cache |
+| `camptickets-npm` → `/root/.npm` | npm download cache |
+
+For this to work the runner must **allow** these volumes via `config.yaml`
+(`container.valid_volumes`) — `deploy-runner.sh` writes that config, pre-creates
+the volumes, and starts the daemon with `--config /data/config.yaml`. If you set
+the runner up by hand, copy `config.yaml` next to the compose file and add
+`--config /data/config.yaml` to the daemon command.
+
+To force a clean rebuild, remove the volumes:
+`docker volume rm camptickets-android-sdk camptickets-gradle camptickets-npm`.
+
 ## Notes
 
 - The runner I initially registered on the app host has been removed. If Forgejo
@@ -37,3 +58,6 @@ The runner advertises the `docker` label; the build workflow (`.forgejo/workflow
   *Settings → Actions → Runners*.
 - `DOCKER_GID` must match the roomy server's docker socket group, or the runner
   can't reach the Docker daemon.
+- After updating to a caching-enabled runner, re-run `deploy-runner.sh` (or
+  `docker compose up -d` in the runner dir) so the new `config.yaml` + volumes
+  take effect, then push a fresh tag.

runner/config.yaml

@@ -0,0 +1,9 @@
+# Forgejo runner config. The important bit is valid_volumes: it whitelists the
+# named Docker volumes the build-apk workflow mounts as persistent caches
+# (Android SDK/NDK, Gradle home, npm cache). Without these listed, the runner
+# rejects the workflow's `volumes:` and the job fails.
+container:
+  valid_volumes:
+    - camptickets-android-sdk
+    - camptickets-gradle
+    - camptickets-npm

runner/docker-compose.yml

@@ -11,6 +11,9 @@ services:
       - "${DOCKER_GID:-988}"
     volumes:
       - ./data:/data
+      # config.yaml allows the build workflow to mount the persistent cache
+      # volumes (valid_volumes); without it those mounts are rejected.
+      - ./config.yaml:/data/config.yaml:ro
       # Runner spawns job containers via the host Docker daemon.
       - /var/run/docker.sock:/var/run/docker.sock
     env_file: .env
@@ -31,4 +34,4 @@ services:
             --name "${RUNNER_NAME:-camptickets-runner}" \
             --labels "docker:docker://node:22-bookworm"
         fi
-        exec forgejo-runner daemon
+        exec forgejo-runner daemon --config /data/config.yaml