docker: adapt to latest python:3.8-slim-buster

[tiborsimko]

When building the image with latest python:3.8-slim-buster, the build times were unusually slow because some of the dependencies wanted to bring over Python 2 instead of Python 3 libraries. This commit fixes the problem and makes the build process fast again.

(However, Python 3.7 system packages are still being brought, which is inconsistent with Python 3.8 version that the image provides. An upgrade to Python 3.9 or a replacement of the base image may be wanted soon.)

[tiborsimko]

How to reproduce the problem:

$ cd reana-workflow-engine-yadage
$ git checkout master
$ git pull python:3.8-slim-buster
$ time reana-dev docker-build -c . --no-cache

(and see the last command taking a long time...)

This branch fixes the problem and makes the build fast again:

$ git checkout faster-build
$ time reana-dev docker-build -c . --no-cache

[codecov-commenter]

Codecov Report

Merging #230 (d7927b6) into master (ec04ec0) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #230   +/-   ##
=======================================
  Coverage   44.92%   44.92%           
=======================================
  Files           6        6           
  Lines         256      256           
=======================================
  Hits          115      115           
  Misses        141      141