Make the `make init-docker` faster

[funwarioisii]

The make init-docker takes a lot of time.
This is detrimental to developer experience.
I think the make init-docker should be faster.

(Currently no idea 😇 )

[takahi-i]

How about using the image docker for Python 3.8 in Docker hub.

We can reduce steps to install python https://github.com/docker-science/cookiecutter-docker-science/blob/master/%7B%7B%20cookiecutter.project_slug%20%7D%7D/docker/Dockerfile#L7-L11.