add mkdir line

.devcontainer/Dockerfile

@@ -16,6 +16,7 @@ ARG USER_GID=$USER_UID
 RUN mkdir -p /workspace/growi/node_modules
 RUN mkdir -p /workspace/growi/packages/app/node_modules
 RUN mkdir -p /workspace/growi/packages/slackbot-proxy/node_modules
+RUN mkdir -p /workspace/growi/packages/app/.next
 
 # [Optional] Update UID/GID if needed
 RUN if [ "$USER_GID" != "1000" ] || [ "$USER_UID" != "1000" ]; then \