Hello @rjohnson,
Just to clarify, you need to access your REACT_APP_BACKEND_URL env variable from your Dockerfile, is that what you are asking?
By default, all env variables are injected into your container hence accessible at runtime.
If you need to access this env var at build time (in Dockerfile) you need to explicitly declare this variable as ARG in your Dockerfile, you can check this post explaining it.
Let me know if it helps,
Cheers