python: command not found

[zccoder]

While i pulled the docker, and run the demo, it occurs the 'python: command not found' problem.
It seems that the python version in the docker is not right.

[zccoder]

I found the python version in the docker is 3.5, while it should be python >=3.6 to support some expressions.
We should upgrade the python version, however, it means that all packages in the dockers all should be reinstalled. So I think it is not a good idea to install by docker.