fix: give nextjs user a valid shell for su

--system flag creates the user with /sbin/nologin, causing
'This account is not available' when entrypoint tries su.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -23,7 +23,7 @@ ENV NEXT_TELEMETRY_DISABLED=1
 ENV DB_PATH=/app/data/zombie_invasion.db
 
 RUN addgroup --system --gid 1001 nodejs && \
-    adduser --system --uid 1001 nextjs
+    adduser --system --uid 1001 --shell /bin/sh nextjs
 
 COPY --from=builder /app/.next/standalone ./
 COPY --from=builder /app/.next/static ./.next/static