It isn't clear what you're trying to as you're talking about iDRAC and VNC. So you have a Docker container running what? Linux? And you're trying to access what? I am going to presume you want to access the Enterprise iDRAC's virtual console screen.
Now, you mention iDRAC6. This is for a gen11 server, is that correct... so a Rx10 PowerEdge server like a R710 or something? The console on iDRAC6 is a Java based console, not VNC. You have to have Java installed on the machine you're running your browser. And it needs to be java-8-jre if I remember right. Newer iDRACs don't have this issue.
I don't think you need to be using Docker unless you can't install the JRE on your host. And then, I am not sure how you'd be using it from inside a container as you'd need to be running your browser from there. Sorry if I misunderstand.
I can tell you with absolute certainty (because we do it in production environments), the iDRAC6 console needs a specific old version of Java.
Fill in more details, I'm happy to help you if I can. It's a bugger to get it working, but it is possible.