Remove WPR_DOMAIN_NAME parameter

[KharchenkoMaks]

Previously, the container used WPR_DOMAIN_NAME to set the nginx server_name directive. However, with a single default_server block, server_name has no effect — the server responds to all domains regardless. The parameter was removed as it provided no real access control and created a false sense of security.

Test plan:

• Verify build with default parameters. Check that the container accepts requests from any domain and demo pages work.
• Verify that passing --env WPR_DOMAIN_NAME=... at runtime is silently ignored (no errors).
• Verify HTTPS startup: self-signed certificate is generated with CN=localhost.