Refactor the Dockerfiles

[edsoncelio]

Everyone,
I noticed that many of the dockerfiles are not optimized or with best practices.
I can contribute to this task (like this #189 ), but I would like to know if if it is a valid contribution.

Regards,

[rramacciani]

Hi,

I've just created a fork (I don't have the permission to open a pull request) in order to optimize the mongodb/centos7/Dockerfile

Regards