Force docker to pull FROM images when building

[dpordomingo]

After fixing MariaDB issue #279 we found again that libmariadbclient18 was missing in some others mate's PCs.

Turns out that libmariadbclient18 was removed from debian buster (base of python:3.6-slim), but when building the image with an old version of python:3.6 and python:3.6-slim, it failed.

The solution appeared deleting current python:3.6 and python:3.6-slim images, and building again, which pulled the proper (and new) versions of python:3.6 and python:3.6-slim.

Adding --pull in our docker build commands will do it by default, to avoid building using old versions of our FROM images.

some info: https://www.databasesandlife.com/docker-build-pull-option

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• I have updated the CHANGELOG file according to the conventions in keepachangelog.com
• This PR contains changes that do not require a mention in the CHANGELOG file

[se7entyse7en]

Given that --pull means to Always attempt to pull a newer version of the image this could happen again for future versions of debian. Isn't it better to just change the base images as follows?

• python:3.6 -> python:3.6-buster
• python:3.6-slim -> python:3.6-slim-buster

[dpordomingo]

Good idea @se7entyse7en
Travis running with that locked images from bae02c5