fix Dockerfile: aggiorna Java 17 → 21 per Capacitor Android

Capacitor Android richiede source release 21, OpenJDK 17 causava
"error: invalid source release: 21" durante compileDebugJavaWithJavac.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/Dockerfile

@@ -3,7 +3,7 @@ FROM node:20-slim
 
 # Dipendenze di sistema
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    openjdk-17-jdk-headless \
+    openjdk-21-jdk-headless \
     wget \
     unzip \
     && rm -rf /var/lib/apt/lists/*