Fix Docker build

[sirodoht]

This PR adjusts Docker base image from python:3.6.8-slim to python:3.6 because debian no longer provide apt repositories for debian stretch slim version:

1.034 W: The repository 'http://security.debian.org/debian-security stretch/updates Release' does not have a Release file.
1.034 W: The repository 'http://deb.debian.org/debian stretch Release' does not have a Release file.
1.034 W: The repository 'http://deb.debian.org/debian stretch-updates Release' does not have a Release file.
1.034 E: Failed to fetch http://security.debian.org/debian-security/dists/stretch/updates/main/binary-arm64/Packages  404  Not Found
1.034 E: Failed to fetch http://deb.debian.org/debian/dists/stretch/main/binary-arm64/Packages  404  Not Found
1.034 E: Failed to fetch http://deb.debian.org/debian/dists/stretch-updates/main/binary-arm64/Packages  404  Not Found

Also, django-flatpages-tinymce is removed as it's no longer compatible with other requirements:

1.432 ERROR: Could not find a version that satisfies the requirement django-flatpages-tinymce==0.1.1 (from versions: none)
1.432 ERROR: No matching distribution found for django-flatpages-tinymce==0.1.1

I don't know where django-flatpages-tinymce is used (it will no longer work) but this can be solved if we have a working dev build.