add http health checks.

We normally use "grace period" to mean the time between notification and SIGKILL. We should be passing this as the "t" parameter to Docker's stop operation.

If this is the probe interval, we should call it that. Also, any time-related fields should specify the units in the name, such as probeIntervalSeconds.

And, there almost certainly needs a startup-time allowance of some sort, such as a different initial probe interval, an additional initial probe delay, or a number of initial probes that are allowed to fail without consequence. I'm fine if this gets pushed to a subsequent PR, because adding it wouldn't break compatibility, especially if one of the latter 2 approaches were taken.