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The key here was to build frontend-platform before trying to build frontent-app-library-authoring.
VS Code’s Dev Containers Plugin AUTOMATICALLY started port forwarding
I was connecting to the CMS container, and didn’t realize that VS Code would automatically start port forwarding. This is a problem because Django in the CMS container runs at 8000, but is supposed to be connected to at 8001 from the outside. Port 8000 is reserved the for LMS. So what was happening was that VS Code was taking the port away from the LMS whenever the LMS code was reloaded. I was working on a Studio feature, so things continued to work fine for me until my auth token expired and the Library Authoring MFE errored because it was trying to hit an oauth2 endpoint that lives on the LMS, but was forwarded to CMS instead.
I haven’t seen a way to turn this off in the settings, but being able to connect to a container and get all the python code loaded for introspection is really nice. So I’ve taken to always connecting to the LMS container–VS Code still grabs and forwards the port, but it forwards it to the same place it was going before.