Base python:slim on debian:stretch-slim

[chpio]

Hi,

1. strech is the current stable release and (IMHO) should be used as default.
2. python:slim is based on a non slim debian image, which is kind of misleading given the name python:slim

[douglasmiranda]

Related: #104

[yosifkit]

1. since we don't have an simple way to let users know when the default for python:3.6 will be stretch rather than jessie we have not made a decision when to switch over since that is a large breaking change for many

   • since Add explicit "jessie" and "alpine3.4" aliases #215 we do label the images so that users can choose jessie or stretch for python 2.7 and 3.6 (ex python:2.7-stretch and python:3.6-stretch), though we do not yet have a slim variant for the stretch variety
   • stretch is the default in python 3.7 and above (currently in alpha stages)
   • I think adding a python:3.6-slim-stretch would be useful (along with explicit -jessie and -wheezy on the rest of the -slim images), but until we have a defined policy regarding Distro updates we cannot change default distribution in minor releases. (similar work to Update base images to Debian Stretch and Alpine 3.6 php#504)

2. the slim in python images is about the size of the image and does not necessarily indicate being from a Debian slim (the python slim came first, there was no Debian slim)

   This image does not contain the common packages contained in the default tag and only contains the minimal packages needed to run python. Unless you are working in an environment where only the python image will be deployed and you have space constraints, we highly recommend using the default image of this repository.

   - https://github.com/docker-library/docs/tree/212ed60a19b8b5454308b504fdcdf5d317f42b6f/python#pythonslim

   • if this will not break users, I am not opposed to moving the slim python images to be from the slim Debian images