fix Dockerfile and Change to python3

[matsubaraDaisuke]

Hi I'm matsubara.

I tried to bug-fix of dockerfile

The bug is closed issue because of outer commit.
#175

MacBook-Pro:PyGEM2 matsubaradaisuke$ docker-compose run   pygem /bin/bash
PyGeM@219e5e098ca3:~$ Mar 24 12:29:44 219e5e098ca3 syslog-ng[16]: syslog-ng starting up; version='3.5.6'

PyGeM@219e5e098ca3:~$ python3.7
Python 3.7.6 (default, Jan  8 2020, 19:59:22)
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pygem
>>>

[mtezzele]

Dear @matsubaraDaisuke, thank you for the contribution!