Fix case where health start interval is 0 uses default

When the start interval is 0 we should treat that as unset.
This is especially important for older API versions where we reset the
value to 0.

Instead of using the default probe value we should be using the
configured `interval` value (which may be a default as well) which gives
us back the old behavior before support for start interval was added.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/health.go

@@ -248,7 +248,7 @@ func handleProbeResult(d *Daemon, c *container.Container, result *types.Healthch
 // There is never more than one monitor thread running per container at a time.
 func monitor(d *Daemon, c *container.Container, stop chan struct{}, probe probe) {
 	probeInterval := timeoutWithDefault(c.Config.Healthcheck.Interval, defaultProbeInterval)
-	startInterval := timeoutWithDefault(c.Config.Healthcheck.StartInterval, defaultProbeInterval)
+	startInterval := timeoutWithDefault(c.Config.Healthcheck.StartInterval, probeInterval)
 	startPeriod := timeoutWithDefault(c.Config.Healthcheck.StartPeriod, defaultStartPeriod)
 
 	c.Lock()