What is this 'Docker' metric? Am I about to run out of something? I should have plenty ram but looks like this 'Docker' thing is not picking up on that. 

It's the percentage of the docker.img file that is in use. If it starts growing rapidly that usually means a container is not configured properly. If it's been close to that figure for a while and is slowly increasing a little only when you add new containers, it's probably normal.

 

With the docker service stopped you can increase the size of the image file, but don't just increase it if you can't point to a reason it's getting larger, if something is configured incorrectly just making the image bigger delays the inevitable troubleshooting.

 

It's got absolutely nothing to do with RAM.