Use Miniconda3 version 4.3.21 rather than latest

[maartenterpstra]

The problem

The "latest" version of Miniconda3 is based on Python 3.7, which is incompatible with Djano 1.11.3. When this version of Miniconda3 is used, the Docker images cannot be started because an exception like the following is thrown:

web_1     | SyntaxError: Generator expression must be parenthesized
web_1     | Traceback (most recent call last):
web_1     |   File "manage.py", line 22, in <module>
web_1     |     execute_from_command_line(sys.argv)
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
web_1     |     utility.execute()
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/core/management/__init__.py", line 337, in execute
web_1     |     django.setup()
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/__init__.py", line 27, in setup
web_1     |     apps.populate(settings.INSTALLED_APPS)
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/apps/registry.py", line 85, in populate
web_1     |     app_config = AppConfig.create(entry)
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/apps/config.py", line 94, in create
web_1     |     module = import_module(entry)
web_1     |   File "/miniconda/lib/python3.7/importlib/__init__.py", line 127, in import_module
web_1     |     return _bootstrap._gcd_import(name[level:], package, level)
web_1     |   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
web_1     |   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
web_1     |   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
web_1     |   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
web_1     |   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
web_1     |   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/contrib/admin/__init__.py", line 4, in <module>
web_1     |     from django.contrib.admin.filters import (
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/contrib/admin/filters.py", line 10, in <module>
web_1     |     from django.contrib.admin.options import IncorrectLookupParameters
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/contrib/admin/options.py", line 12, in <module>
web_1     |     from django.contrib.admin import helpers, widgets
web_1     |   File "/miniconda/lib/python3.7/site-packages/django/contrib/admin/widgets.py", line 151
web_1     |     '%s=%s' % (k, v) for k, v in params.items(),

Proposed fix

By changing to Miniconda3 latest to Miniconda3 4.3.21 (which I assume was "latest" at project inception based on the Django version), Python 3.6 is enforced and the Docker image can actually be built and started.