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.
This may confuse people; if I understand your proposal correctly, the sleep time will *not* be reset if you specify a fixed interval (e.g. `2s`), so this only applies to the default behavior.

Maybe move the "default" behavior (incrementing interval) to a separate paragraph to make the distinction clearer?