fix: exclude admin src from Docker context but keep package.json for workspace resolution

.dockerignore

@@ -6,6 +6,7 @@ planning
 deploy
 infra
 packages/admin
+!packages/admin/package.json
 Dockerfile*
 docker-compose*
 *.md

Dockerfile

@@ -3,7 +3,8 @@ WORKDIR /app
 COPY package.json bun.lock ./
 COPY packages/shared/package.json packages/shared/
 COPY packages/backend/package.json packages/backend/
-RUN bun install --frozen-lockfile --filter @lunarfront/backend
+COPY packages/admin/package.json packages/admin/
+RUN bun install --frozen-lockfile
 
 FROM oven/bun:1.3.11-alpine AS build
 ARG APP_VERSION=dev