fix: switch from Alpine to Debian slim Node.js image to resolve SWC binary loading issue

- Changed FROM node:20-alpine to node:20-slim for both builder and runner stages
- Alpine (musl) is incompatible with Next.js SWC binary (glibc)
- Debian slim provides 80% smaller image compared to full node:20 (~180MB vs ~900MB)
- Fully compatible with Next.js and SWC without any patches
- Better performance and stability on Synology NAS

frontend/Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM node:20-alpine AS builder
+FROM node:20-slim AS builder
 
 WORKDIR /app
 
@@ -15,7 +15,7 @@ ENV NEXT_TELEMETRY_DISABLED=1
 RUN npm run build
 
 # Production stage
-FROM node:20-alpine AS runner
+FROM node:20-slim AS runner
 
 WORKDIR /app