Fix debian (9) stretch release issue in docker image

[sumandari]

Background:

I encountered this issue when running make build on my local:

 => ERROR [ 3/16] RUN apt-get update && apt-get install -y libsasl2-dev python-dev libldap2-dev li  1.7s
------                                                                                                   
 > [ 3/16] RUN apt-get update && apt-get install -y libsasl2-dev python-dev libldap2-dev libssl-dev:     
#0 0.472 Ign:1 http://security.debian.org/debian-security stretch/updates InRelease                      
#0 0.519 Ign:2 http://deb.debian.org/debian stretch InRelease
#0 0.519 Err:3 http://security.debian.org/debian-security stretch/updates Release
#0 0.519   404  Not Found
#0 0.550 Ign:4 http://deb.debian.org/debian stretch-updates InRelease
#0 0.583 Err:5 http://deb.debian.org/debian stretch Release
#0 0.583   404  Not Found
#0 0.616 Err:6 http://deb.debian.org/debian stretch-updates Release
#0 0.616   404  Not Found
#0 1.103 Get:7 https://deb.nodesource.com/node_10.x stretch InRelease [4585 B]
#0 1.258 Get:8 https://deb.nodesource.com/node_10.x stretch/main amd64 Packages [767 B]
#0 1.281 Reading package lists...
#0 1.670 E: The repository 'http://security.debian.org/debian-security stretch/updates Release' does no longer have a Release file.
#0 1.670 E: The repository 'http://deb.debian.org/debian stretch Release' does no longer have a Release file.
#0 1.670 E: The repository 'http://deb.debian.org/debian stretch-updates Release' does no longer have a Release file.
------
Dockerfile:8
--------------------
   6 |     #RUN  ln -s /bin/true /sbin/initctl
   7 |     RUN apt-get clean all
   8 | >>> RUN apt-get update && apt-get install -y libsasl2-dev python-dev libldap2-dev libssl-dev
   9 |     ADD REQUIREMENTS.txt /REQUIREMENTS.txt
  10 |     RUN pip install -r /REQUIREMENTS.txt
--------------------

Proposed solution:

There was a discussion on https://serverfault.com/questions/1074688/security-debian-org-does-not-have-a-release-file-on-with-debian-docker-images.

This PR is to use stretch main contrib non-free package repository. It did fix my issue, however, I'm not sure if the non-free package can be used in this project.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -38.64 :warning:

Comparison is base (7363004) 76.34% compared to head (ccd0ca4) 37.71%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #278 +/- ## =========================================== - Coverage 76.34% 37.71% -38.64% =========================================== Files 126 90 -36 Lines 4764 3776 -988 =========================================== - Hits 3637 1424 -2213 - Misses 1127 2352 +1225 ``` [see 72 files with indirect coverage changes](https://app.codecov.io/gh/qgis/QGIS-Django/pull/278/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qgis)

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[dimasciput]

Thanks @sumandari