Add temporary "escape hatch" envvar for setuptools v50.0.0

Without this workaround we just started receiving the
following error during build now.

```
Traceback (most recent call last):
  File "manage.py", line 8, in <module>
    from django.core.management import execute_from_command_line
  File "/usr/local/lib/python3.5/dist-packages/django/__init__.py", line 1, in <module>
    from django.utils.version import get_version
  File "/usr/local/lib/python3.5/dist-packages/django/utils/version.py", line 6, in <module>
    from distutils.version import LooseVersion
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 666, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 577, in module_from_spec
  File "/usr/local/lib/python3.5/dist-packages/_distutils_hack/__init__.py", line 82, in create_module
    return importlib.import_module('._distutils', 'setuptools')
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 981, in _gcd_import
  File "<frozen importlib._bootstrap>", line 931, in _sanity_check
SystemError: Parent module 'setuptools' not loaded, cannot perform relative import
```

see [setuptools changelog](https://github.com/pypa/setuptools/blob/17cb9d6bf249cefe653d3bdb712582409035a7db/CHANGES.rst#v5000)
and this [setuptools issue](pypa/setuptools#2352) for
details

Dockerfile

@@ -55,6 +55,10 @@ ENV APP_GIT_COMMIT=${COMMIT_ID}
 ENV APP_BUILD_DATE=${BUILD_DATE}
 ENV APP_BUILD_TAG=${BUILD_TAG}
 
+# temporary "escape hatch" for setuputools > 50.0
+# https://github.com/pypa/setuptools/issues/2352
+ENV SETUPTOOLS_USE_DISTUTILS=stdlib
+
 # clearing data to prevent UNIQUE constraint violations when rebuilding locally, at least
 RUN python3 manage.py migrate --no-input \
     && python3 manage.py cleardata \