ci: cut PR CI time with a prebuilt image, ccache, and no QML cachegen (#22)

CI took about 5 min 10 s per PR on vm-201: 1.5 min of `pacman -Syu` and 3.5 min of build, half of it qmlcachegen compiling QML the tests never load. Now it takes **1 min 49 s with a cold ccache and 26 s with a warm one**.

- CI runs in a prebuilt Arch image with the build deps (`git.trivk.dev/tri/qtpotato-ci:latest`). `ci-image.yml` rebuilds it weekly, when its Dockerfile changes, or on demand. It builds on the runner's own Docker daemon, so vm-201 has the new image right away, since the runner does not re-pull tags it already has.
- CI configures with `QT_QML_NO_CACHEGEN=ON` and no debug info. Release and AUR builds keep both.
- ccache, kept between runs with `actions/cache`.
- Docs-only changes (`README.md`, `LICENSES/`) skip CI.

Pushing the image to the registry needed two workarounds. Gitea's Actions token cannot push container images, so it uses the `PACKAGE_TOKEN` secret from #21. Cloudflare rejects request bodies over 100 MB and `docker push` sends each layer in one request, so `regctl` (pinned by sha256) pushes the layers in 50 MB chunks.

Giving vm-201 more vCPUs is left out; it is an infra change.

Refs #20

Proof: on this PR, CI run 328 (cold ccache) took 1 min 49 s and run 332 (warm, 41/44 hits) took 26 s, both 8/8 tests. Image run 333 pushed the image, and the registry digest matches the runner's local image. A local build with the CI flags takes 74 steps instead of 154.

Reviewed with sol-loop: 3 rounds with gpt-6.1-sol. Sol verdict: approved

Created with Claude Opus 5.5 in Claude Code (T3 Code).
Reviewed-on: #22
Co-authored-by: Tri Vo Khac <git@trivk.dev>

.gitea/ci-image/Dockerfile

@@ -0,0 +1,7 @@
+# Arch Linux with QtPotato's build dependencies, so CI skips the pacman step.
+# Built and pushed by .gitea/workflows/ci-image.yml.
+FROM archlinux:latest
+RUN pacman -Syu --noconfirm --needed git nodejs base-devel cmake ninja ccache extra-cmake-modules \
+        qt6-base qt6-declarative kirigami ki18n kcoreaddons kcrash kconfig \
+        mpvqt qcoro qtkeychain-qt6 ffmpeg \
+    && rm -rf /var/cache/pacman/pkg/*

.gitea/workflows/ci-image.yml

@@ -0,0 +1,40 @@
+name: CI image
+
+# Builds the Arch image that ci.yml runs in, with the build dependencies preinstalled.
+# Rebuilt weekly to pick up Arch updates.
+on:
+  push:
+    branches: [main]
+    paths: ['.gitea/ci-image/**', '.gitea/workflows/ci-image.yml']
+  schedule:
+    - cron: '0 3 * * 1'
+  workflow_dispatch:
+
+jobs:
+  image:
+    # The build runs on the runner's own Docker daemon, so the amd64 runner has the new
+    # image locally right away. It does not re-pull tags it already has.
+    runs-on: amd64
+    env:
+      IMAGE: git.trivk.dev/tri/qtpotato-ci:latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        # No layer cache, so each rebuild picks up the week's Arch updates.
+        run: docker build --pull --no-cache -t "$IMAGE" .gitea/ci-image
+      - name: Push
+        # Gitea's Actions token cannot push to the container registry, so this uses an
+        # access token with the write:package scope. Cloudflare rejects request bodies over
+        # 100 MB and docker push sends each layer in one request, so regctl pushes the
+        # layers in 50 MB chunks instead.
+        env:
+          TOKEN: ${{ secrets.PACKAGE_TOKEN }}
+        run: |
+          curl -fsSL -o /usr/local/bin/regctl \
+              https://github.com/regclient/regclient/releases/download/v0.11.7/regctl-linux-amd64
+          echo "3737fbbdd791e04e7a831640b681f3397a376498e80e2ad9b1b2d27296f239bc  /usr/local/bin/regctl" | sha256sum -c
+          chmod +x /usr/local/bin/regctl
+          echo "$TOKEN" | regctl registry login git.trivk.dev -u "${{ github.repository_owner }}" --pass-stdin
+          regctl registry set git.trivk.dev --blob-chunk 52428800 --blob-max 52428800
+          docker save "$IMAGE" -o "$RUNNER_TEMP/image.tar"
+          regctl image import "$IMAGE" "$RUNNER_TEMP/image.tar"

.gitea/workflows/ci.yml

@@ -3,24 +3,39 @@ name: CI
 on:
   push:
     branches: [main]
+    paths-ignore: ['README.md', 'LICENSES/**']
   pull_request:
+    paths-ignore: ['README.md', 'LICENSES/**']
 
 jobs:
   build:
     # Arch Linux only ships x86_64 images, so pick an amd64 runner.
     runs-on: amd64
-    container: archlinux:latest
+    # Arch with the build dependencies preinstalled; see ci-image.yml.
+    container: git.trivk.dev/tri/qtpotato-ci:latest
+    env:
+      CCACHE_DIR: ${{ github.workspace }}/.ccache
+      # Keep hits across weekly image rebuilds when the compiler did not change.
+      CCACHE_COMPILERCHECK: content
+      CCACHE_MAXSIZE: 500M
     steps:
-      - name: Install dependencies
-        run: |
-          pacman -Syu --noconfirm --needed git nodejs base-devel cmake ninja extra-cmake-modules \
-              qt6-base qt6-declarative kirigami ki18n kcoreaddons kcrash kconfig \
-              mpvqt qcoro qtkeychain-qt6 ffmpeg
       - uses: actions/checkout@v4
+      - uses: actions/cache@v4
+        with:
+          path: .ccache
+          key: ccache-${{ github.sha }}
+          restore-keys: ccache-
       - name: Configure
-        run: cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTING=ON
+        # The tests do not load QML, so skip QML cachegen and debug info: they take over
+        # half of the build. Release and AUR builds keep both.
+        run: |
+          cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS_DEBUG=-O0 \
+              -DQT_QML_NO_CACHEGEN=ON -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DBUILD_TESTING=ON
       - name: Build
-        run: cmake --build build
+        run: |
+          ccache -z
+          cmake --build build
+          ccache -s
       - name: Test
         env:
           QT_QPA_PLATFORM: offscreen