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/static_pool_test.go

@@ -24,6 +24,10 @@ func (r *mockStaticRunner) TryExec(ctx context.Context, call pool.RunnerCall) (b
 	return true, nil
 }
 
+func (r *mockStaticRunner) Status(ctx context.Context) (*pool.RunnerStatus, error) {
+	return nil, nil
+}
+
 func (r *mockStaticRunner) Close(context.Context) error {
 	return ErrorGarbanzoBeans
 }