Finally, I was able to find where to add the port-forwarding rule. Open the oracle VirtualBox Manager. Go to Settings–>Network –> Port Forwarding..
Here 1 rule is already available for ssh.
So I added another rule for this with below configurations:
HostIP:HostPort=> 127.0.0.1:9999
GuestIP:GuestPort=> :8888
So now if I invoke http://localhost:9999 , it is forwarded to => docker-machine:8888 and from there it is forwarded to => tomcatVM:8080.