feat: removing the never-used Dockerfile-testing

* remove the dockerfile
* remove the appropriate makefile targets
* remove information about the unused testing docker from testing.rst

FIXES: APER-3434

Makefile

@@ -172,18 +172,5 @@ pii_check: ## Check for PII annotations on all Django models
 coverage:
 	coverage xml
 
-### Local testing suite commands ###
-
-build_test_image: # Builds Docker image used for testing so devs don't need to install requirements locally (useful for firefox / xvfb)
-	docker build -t credentials:local -f Dockerfile-testing .
-
-# This should be ran locally, not inside of the devstack container
-quality_and_translations_tests_suite: build_test_image
-	docker run -e "TERM=xterm-256color" -v /edx/app/credentials/node_modules/ -v `pwd`:/edx/app/credentials/ credentials:local bash -c 'cd /edx/app/credentials/ && make check_translations_up_to_date && make validate_translations && make quality && make check_keywords && make pii_check'
-
-# This should be ran locally, not inside of the devstack container
-unit_tests_suite: build_test_image
-	docker run -e "TERM=xterm-256color" -v /edx/app/credentials/node_modules/ -v `pwd`:/edx/app/credentials/ credentials:local bash -c 'cd /edx/app/credentials/ && make static && make tests && make coverage'
-
 docs:
 	tox -e docs

docs/testing.rst

@@ -1,22 +1,6 @@
 Testing
 =======
 
-In order for developers to have a consistent experience between CI and local testing, we are using a locally built
-container image that mimics the ones that Github Actions uses.
-
-NOTE: The first time you run any of the test suites below, it
-will build the image which will take a few minutes. Following test runs will be quicker.
-
-To make testing easier, there are a few commands that mirror our suites in CI:
-
-.. code-block:: bash
-
-    $ make unit_tests_suite
-    $ make quality_and_translations_tests_suite
-
-This will run the Python, the Javascript tests, and our quality and translation suite, respectively.
-
-
 Writing Python Tests
 --------------------
 Tests should be written for all new features. The `Django docs`_ are a good resource for learning how to test Django code.