Why my docker image can not start app when I run it after import

Below is my dockerfile

FROM node:10.15.0
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY ./build/release /usr/src/app/
RUN yarn
EXPOSE 3000
CMD [ "node", "server.js" ]

First I ran

docker build -t app .

and then

docker run -t -p 3000:3000 app

Everything works fine via localhost:3000 in my computer.

Then I try to export this image by

 docker export 68719e2bb0cd > app.tar

and import again by

cat app.tar | docker import - app2

then run
docker run -t -d -p 2000:3000 app2

and the error came out

docker: Error response from daemon: No command specified.

Why this happened?

Source: StackOverflow