chore(docker): enhance Dockerfile for backend to manage nested dependencies

Updated the Dockerfile to create a directory for nested workspace dependencies and adjusted the copy commands to ensure proper handling of node_modules during the build process.

backend/Dockerfile

@@ -31,7 +31,9 @@ COPY packages/contracts packages/contracts
 COPY backend backend
 RUN npm run build -w @mmapp/contracts \
   && npm run build -w mikrotik-manager-backend \
-  && npm prune --omit=dev
+  && npm prune --omit=dev \
+  # npm may nest workspace deps (e.g. dotenv) under backend/node_modules — keep dir for COPY
+  && mkdir -p backend/node_modules
 
 FROM node:22-bookworm-slim AS runner
 WORKDIR /app
@@ -45,6 +47,8 @@ COPY --from=build /app/node_modules ./node_modules
 COPY --from=build /app/packages/contracts ./packages/contracts
 COPY --from=build /app/backend/dist ./backend/dist
 COPY --from=build /app/backend/package.json ./backend/package.json
+# Nested install from lockfile (dotenv etc.) — ESM resolves from /app/backend/dist → ../node_modules
+COPY --from=build /app/backend/node_modules ./backend/node_modules
 RUN mkdir -p /app/data
 EXPOSE 8000
 CMD ["node", "backend/dist/index.js"]