feat: add pre-baked CI build images

- cameleer-build:1 — Maven 3.9 + Java 17 + Node.js 22 + curl/jq/procps/bc
- cameleer-docker-builder:1 — Docker 27 + git/curl/jq

Eliminates per-build tool installation across cameleer3 and cameleer3-server.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: Build Images
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: docker:27
+    steps:
+      - run: apk add --no-cache git
+
+      - run: git clone --depth=1 https://cameleer:${REGISTRY_TOKEN}@gitea.siegeln.net/${GITHUB_REPOSITORY}.git .
+        env:
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+
+      - run: echo "$REGISTRY_TOKEN" | docker login gitea.siegeln.net -u cameleer --password-stdin
+        env:
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push build image
+        run: |
+          docker build -t gitea.siegeln.net/cameleer/cameleer-build:1 build/
+          docker push gitea.siegeln.net/cameleer/cameleer-build:1
+
+      - name: Build and push docker-builder image
+        run: |
+          docker build -t gitea.siegeln.net/cameleer/cameleer-docker-builder:1 docker-builder/
+          docker push gitea.siegeln.net/cameleer/cameleer-docker-builder:1