Fix #13: optimize deploy with multi-stage Containerfile

Move the WASM build into a multi-stage Containerfile so the emscripten
compilation happens inside the Docker build. This eliminates the separate
container action step, enables Docker layer caching for faster rebuilds,
and makes the Containerfile self-contained.

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

.gitea/workflows/deploy.yaml

@@ -17,13 +17,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      # ── Build WASM artifacts inside an Emscripten container ──
-      - name: Build WASM
-        uses: docker://emscripten/emsdk:3.1.51
-        with:
-          args: make web
-
-      # ── Build container image and push to Gitea registry ──
+      # ── Build container image (includes WASM build stage) and push ──
       - name: Build and push container image
         run: |
           IMAGE_TAG="${{ env.IMAGE }}:sha-${GITHUB_SHA::8}"