feat: set EDXAPP_TEST_MONGO_HOST=mongodb in openedx-dev (#1083)

Currently, we are asking devs to set this env var manually before they run tests.
We may as well save them some keystrokes by setting it in the dev image
programmatically.

changelog.d/20240624_105125_kyle_test_mongo_host.md

@@ -0,0 +1 @@
+[Improvement] Set `EDXAPP_TEST_MONGO_HOST` env var in the openedx-dev image so that it no longer needs to be set by hand when running edx-platform unit tests (by @kdmccormick).

docs/tutorials/edx-platform.rst

@@ -151,7 +151,6 @@ Then, run unit tests with ``pytest`` commands::
     # Run tests on common apps
     unset DJANGO_SETTINGS_MODULE
     unset SERVICE_VARIANT
-    export EDXAPP_TEST_MONGO_HOST=mongodb
     pytest common
     pytest openedx
     pytest xmodule

tutor/templates/build/openedx/Dockerfile

@@ -275,6 +275,10 @@ RUN pip install -e "/mnt/{{ name }}"
 # Add ipdb as default PYTHONBREAKPOINT
 ENV PYTHONBREAKPOINT=ipdb.set_trace
 
+# Point unit tests at the MongoDB container
+ENV EDXAPP_TEST_MONGO_HOST={{ MONGODB_HOST }}
+ENV EDXAPP_TEST_MONGO_PORT_NUM={{ MONGODB_PORT }}
+
 # Recompile static assets: in development mode all static assets are stored in edx-platform,
 # and the location of these files is stored in webpack-stats.json. If we don't recompile
 # static assets, then production assets will be served instead.