Proposal: Add the ability to restart a container every X seconds

Currently Docker will sleep 100 * 2^(N-1) milliseconds where N is the number of times it has done a restart.  So this means each restart will sleep twice as long as the last sleep.  So 100 milliseconds, then 200, 400, 800, etc.  The problem is that if you have a container that is failing to start for awhile this sleep time can become hours.  For many things (like a container depending on an external service) it is preferable to just always restart on a fixed interval.  For example, just sleep 2 seconds each time.

The syntax for this will be

`--restart always:2s`

I know more docs need to change so I just documented the syntax I'm proposing.  I'm sure there will be many opinions so as soon as there is an agreement on the syntax, then I'll update all the docs.