There are a few things you can try to resolve this issue:

1. Check if the SQL Server Browser service is running. The SQL Server Browser service listens for incoming requests for Microsoft SQL Server resources and provides information about SQL Server instances installed on the computer.
2. Make sure that TCP/IP connections are enabled for your SQL Server instance and make sure that TCP/IP is enabled. Both of these you can check from the SQL Server Configuration Manager.
3. Check your firewall settings. Make sure that the SQL Server port is open on the firewall. The default port for SQL Server is 1433, but this can be configured during installation.
4. Verify the connection string in your .NET application. Make sure that the Data Source value is correct and includes the correct port number. You can also try using the IP address of the SQL Server instead of the server name.

If you're running your .NET application in a Docker container, make sure that the container is connected to the correct network and can communicate with the SQL Server instance. You can also try exposing the SQL Server port on the host machine and mapping it to the container port using the -p option when running the Docker container.

Best regards, Hesham