ci: build in a prebuilt Arch image with ccache and no QML cachegen

.gitea/workflows/ci-image.yml

@@ -0,0 +1,31 @@
+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:
+
+permissions:
+  contents: read
+  packages: write
+
+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
+        run: docker build --pull -t "$IMAGE" .gitea/ci-image
+      - name: Push
+        run: |
+          echo "${{ secrets.GITHUB_TOKEN }}" | docker login git.trivk.dev -u "${{ github.actor }}" --password-stdin
+          docker push "$IMAGE"