Fix Linux Docker instructions

[madhavajay]

The linux docker install instructions do not run the shell script they only download it. They should also make sure to run as super user if needed.

First we should check if the user is root and warn them that using hagrid in root might cause issues if they run it later without root.

Second, we can check if the user they are using is in the sudo group in the dependency checker on linux to make sure they have permission ahead of time.

If they don't we can tell them installation might fail so they may need to get super user access first.

Then after install we should check if they are in the docker group and if not, ask them to add themselves. Here we will need to test / make sure that either hagrid (through subprocess shells) or if there is some way we can restart the session so that we can access docker from their user without logging out and logging back in again. We can at least give them a meaningful error.