dont build client docker image with hard coded values for CLIENT_DOMAIN and WALLET_BACKEND_SERVER

[stfung77]

What problem does your feature solve?

@jacekn suggest looking into another way to pass these values to the client in order to be able to reuse the built docker images. Currently, REACT_APP_CLIENT_DOMAIN and REACT_APP_WALLET_BACKEND_SERVER and passed as arguments during the docker build step so the client code gets compiled with the proper values. This is required because the compiled files are just static files being served by nginx.

Note: this method is also how the anchor test is currently building images

What would you like to see?

@jacekn suggests trying a env-config.js file similar to whats being done here:
https://github.com/stellar/kube/blob/master/kube001-dev/namespaces/account-viewer-dev/account-viewer-v2-preview-03.yaml