Docker access to external SQL Server

I have created my first DOCKER container that is up and running.
I have managed to mount a volume when I ran the container - so I can access files on the host.

Now I have written a program which - if run on the host - accesses a SQL Server instance successfully.

However, when I run the program inside the docker container - it cannot connect to the same SQL Server instance.

I have tried using the server name, the server ip and even adding ,1433 to the server name and server ip address - but still Docker container cannot connect to the SQL server?

What do i do - is there a special docker run instruction that i must invoke to give the container access to that external SQL Server?