fix: admin public 디렉토리 누락 및 Dockerfile COPY 안전 처리

- apps/admin/public/.gitkeep 추가
- Dockerfile: public 디렉토리 미존재 시에도 COPY 성공하도록 수정

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

Dockerfile

@@ -44,7 +44,9 @@ RUN addgroup --system --gid 1001 nodejs
 RUN adduser --system --uid 1001 nextjs
 WORKDIR /app
 
-COPY --from=builder /app/apps/${APP_NAME}/public ./public
+# Ensure public dir exists (some apps may not have one)
+RUN mkdir -p ./public
+COPY --from=builder /app/apps/${APP_NAME}/public/ ./public/
 COPY --from=builder --chown=nextjs:nodejs /app/apps/${APP_NAME}/.next/standalone ./
 COPY --from=builder --chown=nextjs:nodejs /app/apps/${APP_NAME}/.next/static ./apps/${APP_NAME}/.next/static