I need to point my front-end React application to my backend service both configured in Qovery. I created an Alias for the backend service’s external URL named REACT_APP_BACKEND_URL.
The React application is built using a two stage Dockerfile, that does a Yarn build in the first stage and then Nginx in the second stage.
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.