More booted checks

[thekid]

Currently, the boot label supports the CMD kind, which will run a command inside the container to determine whether it's up and running; and NONE. Like Docker's HEALTHCHECK, this is meant to be extended in the future.

FROM debian:jessie

# LABEL boot NONE
LABEL boot CMD /boot.sh

HEALTHCHECK CMD /health.sh

CMD ...

Ideas for kinds

• HTTP - issues a HTTP request against a given URL, 2xx = sucess, error otherwise (see also Support HTTP health checks moby/moby#25364)
• PORT - waits until a given port is open

Extension point

This switch statement is the correct place for implementing more checks.