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>

Containerfile

@@ -1,7 +1,4 @@
-# JNR Web — static file server for the Emscripten build
-#
-# Prerequisites:
-#   make web          (produces dist-web/)
+# JNR Web — multi-stage build: compile WASM then serve static files
 #
 # Build:
 #   podman build -t jnr-web .
@@ -15,9 +12,21 @@
 #   podman save jnr-web | sudo k3s ctr images import -
 #   sudo k3s kubectl apply -f k8s/
 
+# ── Stage 1: Build WASM artifacts ──
+FROM docker.io/emscripten/emsdk:3.1.51 AS builder
+
+WORKDIR /src
+COPY Makefile ./
+COPY include/ include/
+COPY src/ src/
+COPY assets/ assets/
+COPY web/ web/
+RUN make web
+
+# ── Stage 2: Serve static files ──
 FROM docker.io/joseluisq/static-web-server:2
 
-COPY dist-web/ /public/
+COPY --from=builder /src/dist-web/ /public/
 
 # Disable the default cache-control headers which cache .js for 1 year.
 # The .js and .wasm files must always be fetched together (EM_ASM address