Make URL of sourcify server and repository configurable

[svienot]

Description:

These changes allow to configure the URL of the server and repository services via runtime environment variables:

• the URL of the server and repository can be defined with the $SERVER_URL and $REPOSITORY_SERVER_URL environment variables
• the Docker ui-entrypoint.sh is used to insert a piece of script in the index.html of the select form. This script uses the value of $SERVER_URL and $REPOSITORY_SERVER_URL to set corresponding custom variable in the global window object
• the React code can access these values inside the window object
• the URLs will be set to those values, if the environment variable are defined, and to the values assigned at build time to $REACT_APP_SERVER_URL and $REACT_APP_REPOSITORY_SERVER_URL (respectively) otherwise.

Related issue(s):

Fixes #20

Notes for reviewer:

This is code is already running in the POC server (pointed to by Hashscan staging).