move important info on making baselines and remove fat docker readme

Author: archmoj

CONTRIBUTING.md

@@ -220,6 +220,14 @@ python3 -m pip install kaleido
 python3 -m pip install plotly
 ```
 
+**IMPORTANT:** the `baseline`, `test-image` and `test-export` scripts do **not** bundle the source files before
+running the image tests. We recommend running `npm run watch` or `npm start` in
+a separate tab to ensure that the most up-to-date code is used.
+Also if you are adding a new mock, you may need to re-run `npm start` or `npm run watch`
+to be able to find the new mock in the browser.
+To help ensure valid attributes are used in your new mock(s), please run `npm run test-mock`
+or `npm run test-mock mock_name(s)` after adding new mocks or implementing any new attributes.
+
 If you added new mocks to test/image/mocks folder, to generate draft baselines run
 ```sh
 python3 test/image/make_baseline.py = mockFilename1 mockFilename2