ci: migrate caddy build to GitHub Actions

- Move build-caddy workflow from Gitea to GitHub Actions
- Make caddy_plugin optional, supporting comma-separated plugins
- Add README documenting workflow usage

.github/workflows/build-caddy.yaml

@@ -0,0 +1,43 @@
+name: Build Customised Caddy
+
+on:
+  workflow_dispatch:
+    inputs:
+      caddy_version:
+        description: 'Specify Caddy version (e.g., latest, v2.8.4)'
+        required: true
+        default: 'latest'
+      caddy_plugin:
+        description: 'Comma-separated plugin module paths, e.g. github.com/caddy-dns/cloudflare, github.com/caddyserver/nginx-adapter. Leave empty to build vanilla Caddy.'
+        required: false
+
+jobs:
+  build-caddy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Initialise Go environment
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.22'
+          check-latest: true
+
+      - name: Install xcaddy programme
+        run: go install github.com/caddyserver/xcaddy/cmd/xcaddy@latest
+
+      - name: Compile customised Caddy binary
+        run: |
+          PLUGIN_ARGS=""
+          IFS=',' read -ra PLUGINS <<< "${{ inputs.caddy_plugin }}"
+          for PLUGIN in "${PLUGINS[@]}"; do
+            PLUGIN="${PLUGIN// /}"
+            if [ -n "$PLUGIN" ]; then
+              PLUGIN_ARGS="$PLUGIN_ARGS --with $PLUGIN"
+            fi
+          done
+          xcaddy build ${{ inputs.caddy_version }} $PLUGIN_ARGS
+
+      - name: Upload compiled artefact
+        uses: actions/upload-artifact@v3
+        with:
+          name: caddy-custom-linux-amd64
+          path: caddy