feat(ci): Cache and start Supabase in CI workflow

- Cache Supabase Docker images (/var/lib/docker) using actions/cache
  with a key based on supabase/config.toml
- Start Supabase locally in the background and export API URL,
  anon key, and service role key to GITHUB_ENV
- Wait briefly before running the build to allow Supabase to start

Author: mrepol742

.github/workflows/build.yml

@@ -22,5 +22,26 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Cache Supabase Docker images
+        uses: actions/cache@v4
+        with:
+           path: /var/lib/docker
+           key: ${{ runner.os }}-supabase-docker-${{ hashFiles('supabase/config.toml') }}
+           restore-keys: |
+             ${{ runner.os }}-supabase-docker-
+
+      - name: Start Supabase locally in background
+        run: |
+           npx --yes supabase start
+           SUPABASE_URL=$(npx --yes supabase status --output env | awk -F= '/API_URL/ {gsub(/"/,"",$2); print $2}')
+           SUPABASE_ANON_KEY=$(npx --yes supabase status --output env | awk -F= '/PUBLISHABLE_KEY/ {gsub(/"/,"",$2); print $2}')
+           SUPABASE_SERVICE_ROLE_KEY=$(npx --yes supabase status --output env | awk -F= '/SERVICE_ROLE_KEY/ {gsub(/"/,"",$2); print $2}')
+
+           echo "NEXT_PUBLIC_SUPABASE_URL=$SUPABASE_URL" >> $GITHUB_ENV
+           echo "NEXT_PUBLIC_SUPABASE_ANON_KEY=$SUPABASE_ANON_KEY" >> $GITHUB_ENV
+           echo "SUPABASE_SERVICE_ROLE_KEY=$SUPABASE_SERVICE_ROLE_KEY" >> $GITHUB_ENV
+
+           sleep 15 # wait
+
       - name: Build project
         run: npm run build