Healthcheck for `FROM scratch` images.

[reitermarkus]

Currently, we are able to run fwatchdog in a FROM scratch image, but we cannot run the default healthcheck since there is no [/test command to check for a file's existence as far as I can tell.

Possible Solution

We could add a subcommand for fwatchdog which in turn performs the health check instead of relying on a [/test command being present , i.e.

HEALTHCHECK --interval=2s CMD ["/fwatchdog", "healthcheck"]

[alexellis]

Sounds like a fair request. I believe @stefanprodan does similar for his podinfo example.

https://github.com/stefanprodan/podinfo

This would need to be applied to both watchdogs for consistency.