fix(backend): switch base image from alpine to debian for bash compatibility

mvnw is a bash script; eclipse-temurin:21-jdk-alpine only provides ash
(busybox), causing the container to exit silently with code 0 before the
JVM starts. The Debian-based eclipse-temurin:21-jdk image includes bash.

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

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM eclipse-temurin:21-jdk-alpine
+FROM eclipse-temurin:21-jdk
 
 WORKDIR /app