fn: runner status and docker load images (#1116)

* fn: runner status and docker load images

Introducing a function run for pure runner Status
calls. Previously, Status gRPC calls returned active
inflight request counts with the purpose of a simple
health checker. However this is not sufficient since
it does not show if agent or docker is healthy. With
this change, if pure runner is configured with a status
image, that image is executed through docker. The
call uses zero memory/cpu/tmpsize settings to ensure
resource tracker does not block it.

However, operators might not always have a docker
repository accessible/available for status image. Or
operators might not want the status to go over the
network. To allow such cases, and in general possibly
caching docker images, added a new environment variable
FN_DOCKER_LOAD_FILE. If this is set, fn-agent during
startup will load these images that were previously
saved with 'docker save' into docker.

api/agent/drivers/docker/cookie.go

@@ -25,6 +25,18 @@ type cookie struct {
 	drv *DockerDriver
 }
 
+func (c *cookie) configureMem(log logrus.FieldLogger) {
+	if c.task.Memory() == 0 {
+		return
+	}
+
+	mem := int64(c.task.Memory())
+
+	c.opts.Config.Memory = mem
+	c.opts.Config.MemorySwap = mem // disables swap
+	c.opts.Config.KernelMemory = mem
+}
+
 func (c *cookie) configureFsSize(log logrus.FieldLogger) {
 	if c.task.FsSize() == 0 {
 		return