Dockerfile: Fix collectstatic command (#15035)

Recent changes in awx and/or django ansible base cause the django
collectstatic command to fail when using an empty settings file.
Instead, use the defaults settings file from controller via
DJANGO_SETTINGS_MODULE=awx.settings.defaults

[linux/amd64 builder 13/13] RUN AWX_SETTINGS_FILE=/dev/null
SKIP_SECRET_KEY_CHECK=yes SKIP_PG_VERSION_CHECK=yes
/var/lib/awx/venv/awx/bin/awx-manage collectstatic --noinput --clear
Traceback (most recent call last):
(...)
django.core.exceptions.ImproperlyConfigured: settings.DATABASES is improperly
configured. Please supply the ENGINE value. Check settings documentation for
more details.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

tools/ansible/roles/dockerfile/templates/Dockerfile.j2

@@ -90,7 +90,7 @@ WORKDIR /tmp/src/
 RUN make sdist && /var/lib/awx/venv/awx/bin/pip install dist/awx.tar.gz
 
 {% if not headless|bool %}
-RUN AWX_SETTINGS_FILE=/dev/null SKIP_SECRET_KEY_CHECK=yes SKIP_PG_VERSION_CHECK=yes /var/lib/awx/venv/awx/bin/awx-manage collectstatic --noinput --clear
+RUN DJANGO_SETTINGS_MODULE=awx.settings.defaults SKIP_SECRET_KEY_CHECK=yes SKIP_PG_VERSION_CHECK=yes /var/lib/awx/venv/awx/bin/awx-manage collectstatic --noinput --clear
 {% endif %}
 
 {% endif %}