在Dockerfile中定义了一个env变量，如下：
Dockerfile
env
... ENV MY_ENV dev CMD ["pm2","start", "pm2.json"," --env ${MY_ENV}","--no-daemon"] ...
想在镜像RUN的时候，把env变量给替换成指定的，写出如下：
RUN
docker run -e "MY_ENV=staging" --name pm2_test -p 3500:3500 -d pm2_test:v1
问： 为啥变量不起作用？
This is how it is
CMD pm2 start pm2.json --env ${MY_ENV} --no-daemon
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This is how it is