I have a react app hosted on a node docker container. I run it and pass it an environment variable
docker run -t -e "ADMIN_HOST_ENV=http://myapp.app:443/api" -p 3000:3000 "myapp"
How can I get that "ADMIN_HOST_ENV" environment variable within the app?
If this is just about the node backend, there's already a question with a answer: Read environment variables in Node.js
On the frontend, you'll have several options to get at it, create an endpoint to get it, write it to a .js file that's referenced by the frontend, pass it up as a cookie, keep it backend side and only reference it when you hit the backend, etc.
I check environment variables when I run my server, write them to an environment.js, and then reference that environment.js in my index.html.