Add CI caching

.github/workflows/deploy.yml

@@ -35,8 +35,15 @@ jobs:
           KEY: ${{ github.workspace }}/secret.key
         run: cd bin/dither && bundle exec ruby dither.rb >> dither.log 2>&1 &
 
+      - name: Cache Hugo build
+        uses: actions/cache@v3
+        with:
+          path: ${{ github.workspace }}/.cache
+          key: ${{ runner.os }}-hugo_cache
+
       - name: Generate static site
         env:
+          HUGO_CACHEDIR: ${{ github.workspace}}/.cache
           DITHER_SERVER: http://localhost:4567
         run: |
           source bin/get-last-commit.sh