How to copy files and execute pm2 with docker

I have the follow structure

[vagrant@localhost]$ ls
api         package.json yarn.lock
client      favicon.ico
Dockerfile  ecosystem.config.js  README.md

With follow Dockerfile

FROM keymetrics/pm2-docker-alpine
RUN npm install pm2 -g
ADD . .  # COPY HOW CAN I SEE IF WAS COPY
CMD ["pm2-docker", "ecosystem.config.js"]

So my question is.. how to run this image and keep runner with pm2 ?

docker run -d -p 3000:3000 --name backend backend:v3

[vagrant@localhost swarmbot]$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                     PORTS                      NAMES
f28aa7459527        backend:v3          "pm2-docker ecosys..."   10 minutes ago      Exited (1) 9 minutes ago                              backend

Status Exited .. why?