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.

docs/operating/options.md

@@ -40,6 +40,7 @@ docker run -e VAR_NAME=VALUE ...
 | `FN_MAX_FS_SIZE_MB` | Set this option in MB to pass a `size` option to Docker storage driver. This limits the file system size for all containers on the system. See [Docker storage driver options per container](https://docs.docker.com/engine/reference/commandline/run/#set-storage-driver-options-per-container) documentation for details. | None |
 | `FN_DOCKER_NETWORKS` | Set this option with a list of docker networks for function containers to use. If unset, default docker network is used. | None |
 | `FN_DISABLE_READONLY_ROOTFS` | Set this option to enable writable root filesystem. By default root filesystem is mounted read-only. | None |
+| `FN_DOCKER_LOAD_FILE` | Set this option with an absolute path to a tarball to load a set of docker images during fn server startup. The tarball can be generated using [docker save](https://docs.docker.com/engine/reference/commandline/save/).  | None |
 
 ## Starting without Docker in Docker