Most things that I can think of should have come up with a different error. If you didn't have a c runtime set up then amqsputc would not have run. If there was a network connectivity / port issue then you would have got a host not available error. I am suspecting that you are getting something like an untrusted server error, but you should check client side logs to see how far the sample app got before the error occurred.
One thing that you can try to verify connectivity is : If you have a browser in your Windows Docker image, open up the MQ Console. It will be on https://<MQ Server IP>:9443/ibmmq/console/login.html . You may need to accept a connection to an untrusted server. If you can get in then you have proven connectivity and you may have had to mark the server as trusted in the process. If you can't get into the console, then something is blocking the network traffic.