CI: add custom build container

This moves the jekyll build and gh-pages deployment stage into a
customer Github actions file.

Doing this will allow for us to use any custom jekyll plugins which
can't be used with Github's native build system for GH pages, and
therefore we only need to upload the result of the site generation.

.github/workflows/jekyll-docker.yml

@@ -0,0 +1,39 @@
+name: Jekyll site CI
+
+on:
+  push:
+    branches: [ "main" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Build the site in the jekyll/builder container
+      run: |
+        docker run \
+        -v ${{ github.workspace }}:/srv/jekyll \
+        -v ${{ github.workspace }}/_site:/srv/jekyll/_site \
+        jekyll/builder:latest /bin/bash -c \
+        "apk update && \
+        apk add py3-pygments && \
+        chmod -R 777 /srv/jekyll && \
+        jekyll build --config _config.yml,_config-fvwm2rc.yml"
+    - name: Upload artifact
+      uses: actions/upload-pages-artifact@v3
+      with:
+        path: ${{ github.workspace }}/_site
+  # Deployment job
+  deploy:
+    permissions:
+      pages: write      # to deploy to Pages
+      id-token: write   # to verify deployment from a trusted source
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4