docs(devcontainer): Specify interpreter instructions

CONTRIBUTING.md

@@ -13,6 +13,7 @@ To use our dev container:
 3. Within VSCode, install the Remote Development extension (ms-vscode-remote.vscode-remote-extensionpack)
 4. Open the VSCode command palette (cmd / ctrl + shift + p)
 5. Select `Dev Containers: Rebuild and Reopen in container`. A new VSCode window should open up and it should begin setting up your environment. Once it's done, you can open up a new terminal and start running tests or contributing!
+   - Be sure the correct VSCode Python Interpreter is selected. Additional instructions are provided in yellow at the end of the dev container build logs. In short, you need to make sure you have the Poetry interpreter selected or else you'll be using an interpreter without access to the necessary Python packages.
 6. To test that your environment is set up correctly, open a new terminal and run the command `pytest`. You should be able to run all tests and see them pass. Alternatively, you can open up the testing panel, which looks like a beaker, and click the play button to run all of our tests.
 7. After the initial build, you should now be able to leave and re-enter the container any time without needing to rebuild. To do this, open the command palette and select `Dev Containers: Reopen in container`. This will not rebuild the container if you've done it correctly.