ci: add custom Antora Docker image with built-in Mermaid renderer

Custom image based on node:20-alpine with:
- Antora 3.1 site generator
- asciidoctor-kroki for diagram blocks
- @mermaid-js/mermaid-cli with Chromium for local SVG rendering
- No external Kroki server dependency

The GitHub Actions workflow builds the image from docs/.docker/Dockerfile
then uses it to generate the site. Mermaid diagrams are rendered locally
inside the container.

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

Author: michalharakal

.github/workflows/docs.yml

@@ -24,20 +24,27 @@ permissions:
 jobs:
   build-docs:
     runs-on: ubuntu-latest
-    timeout-minutes: 10
+    timeout-minutes: 15
 
     steps:
       - name: Checkout
         uses: actions/checkout@v6
 
+      - name: Build custom Antora image
+        run: |
+          docker build \
+            -t skainet-antora:local \
+            -f docs/.docker/Dockerfile \
+            docs/.docker/
+
       - name: Build Antora site
         run: |
           docker run --rm \
             -v "${{ github.workspace }}:/antora" \
             --workdir /antora/docs \
-            --entrypoint sh \
-            docker.io/antora/antora:3.1 \
-            -c "npm i asciidoctor-kroki && antora --stacktrace antora-playbook.yml"
+            skainet-antora:local \
+            --stacktrace \
+            antora-playbook.yml
 
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3

docs/.docker/.dockerignore

@@ -0,0 +1,2 @@
+node_modules
+build

docs/.docker/Dockerfile

@@ -0,0 +1,35 @@
+FROM node:20-alpine
+
+LABEL org.opencontainers.image.title="SKaiNET Antora" \
+      org.opencontainers.image.description="Antora site generator with built-in Mermaid rendering" \
+      org.opencontainers.image.source="https://github.com/SKaiNET-developers/SKaiNET-transformers"
+
+# Chromium for mermaid-cli (puppeteer)
+RUN apk add --no-cache chromium font-noto
+
+ENV PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium-browser \
+    PUPPETEER_SKIP_DOWNLOAD=true
+
+WORKDIR /antora
+
+# Install Antora + extensions + mermaid-cli in one layer
+RUN npm i --save-exact \
+      @antora/cli@3.1 \
+      @antora/site-generator@3.1 \
+      asciidoctor-kroki@0.18 \
+      @mermaid-js/mermaid-cli@11 \
+    && npm cache clean --force
+
+# Mermaid-cli config: use installed Chromium, no sandbox (container)
+RUN echo '{ \
+  "executablePath": "/usr/bin/chromium-browser", \
+  "args": ["--no-sandbox", "--disable-gpu", "--disable-dev-shm-usage"] \
+}' > /antora/puppeteer-config.json
+
+# Pre-generate a simple diagram to warm up and verify the stack works
+RUN echo 'graph TD; A-->B;' > /tmp/test.mmd \
+    && npx mmdc -i /tmp/test.mmd -o /tmp/test.svg -p /antora/puppeteer-config.json \
+    && rm /tmp/test.mmd /tmp/test.svg
+
+ENTRYPOINT ["npx", "antora"]
+CMD ["--stacktrace", "antora-playbook.yml"]

docs/antora-playbook.yml

@@ -11,10 +11,10 @@ content:
 asciidoc:
   extensions:
     - asciidoctor-kroki
-
-kroki:
-  server-url: https://kroki.io
-  fetch-diagram: true
+  attributes:
+    # Use local mermaid-cli via Kroki (no external server needed when
+    # built with the custom Docker image in docs/.docker/Dockerfile)
+    kroki-fetch-diagram: true
 
 ui:
   bundle: