doc: improve javascript testing in docker

Some instructions were outdated and/or incorrect. Trying to clean things up.

Test Plan:
* qa-cr
* I tested all these changes but feel free to run the new instructions if you like

Change-Id: Iba15c22aff20e2949c19c95a7e2e5b05e6204614
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/311069
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Aaron Shafovaloff <ashafovaloff@instructure.com>
Reviewed-by: Jacob DeWar <jacob.dewar@instructure.com>
Reviewed-by: Caleb Guanzon <cguanzon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Caleb Guanzon <cguanzon@instructure.com>
Build-Review: Andrea Cirulli <andrea.cirulli@instructure.com>

doc/docker/developing_with_docker.md

@@ -205,16 +205,15 @@ First add `docker-compose/js-tests.override.yml` to your `COMPOSE_FILE` var in
 `.env`. Then prepare that container with:
 
 ```
-docker-compose up -d js-tests
-docker-compose exec js-tests yarn install
+docker-compose run --rm js-tests yarn install
 ```
 
-If you run into issues with that command, either during initial setup or after
+If you run into issues with `yarn install`, either during initial setup or after
 updating master, try to fix it with a `nuke_node`:
 
 ```
-docker-compose exec js-tests ./script/nuke_node.sh
-docker-compose exec js-tests yarn install
+docker-compose run --rm js-tests ./script/nuke_node.sh
+docker-compose run --rm js-tests yarn install
 ```
 
 ### QUnit Karma Tests in Headless Chrome
@@ -222,14 +221,21 @@ docker-compose exec js-tests yarn install
 Run all QUnit tests in watch mode with:
 
 ```
-docker-compose exec js-tests
+docker-compose up js-tests
 ```
 
 Or, if you're iterating on something and want to just run a targeted test file
 in watch mode, set the `JSPEC_PATH` env var, e.g.:
 
 ```
-docker-compose exex -e JSPEC_PATH=spec/coffeescripts/util/deparamSpec.js js-tests
+export JSPEC_PATH=spec/coffeescripts/util/deparamSpec.js
+docker-compose up js-tests
+```
+
+To run a targeted test without watch mode:
+
+```
+docker-compose run --rm -e JSPEC_PATH=spec/coffeescripts/util/deparamSpec.js js-tests yarn test:karma:headless
 ```
 
 ### Jest Tests
@@ -237,14 +243,20 @@ docker-compose exex -e JSPEC_PATH=spec/coffeescripts/util/deparamSpec.js js-test
 Run all Jest tests with:
 
 ```
-docker-compose exec js-tests yarn test:jest
+docker-compose run --rm js-tests yarn test:jest
 ```
 
-Or to run a targeted subset of tests in watch mode, use `test:jest:watch` and
+Or run a targeted subset of tests:
+
+```
+docker-compose run --rm js-tests yarn test:jest ui/features/speed_grader/react/__tests__/CommentArea.test.js
+```
+
+To run a targeted subset of tests in watch mode, use `test:jest:watch` and
 specify the paths to the test files as one or more arguments, e.g.:
 
 ```
-docker-compose exec js-tests yarn test:jest:watch app/jsx/actAs/__tests__/ActAsModal.test.js
+docker-compose run --rm js-tests yarn test:jest:watch ui/features/speed_grader/react/__tests__/CommentArea.test.js
 ```
 
 ## Selenium

docker-compose/js-tests.override.yml

@@ -14,6 +14,7 @@ services:
       - tmp:/usr/src/app/tmp
       - yarn-cache:/home/docker/.cache/yarn
     environment:
+      JSPEC_PATH: ${JSPEC_PATH}
       NODE_ENV: CI
     command: yarn run test:karma:watch:headless
 volumes: