Add examples, more documentation around health checks

[thaJeztah]

Problem description

Healthchecks are a powerful feature, but the documentation is currently sparse. There is some basic information on its usage in the builder documentation, and the flags are included in the docker container run and docker service create command line reference, but no practical examples are included in the documentation.

Suggestions for a fix

Docker 17.05 and up adds additional options for health checks (start period), and I wrote a comment on that pull request to illustrate the functionality which may be useful as a starting point moby/moby#28938 (comment), moby/moby#35881 (comment), and I saw @tomwillfixit has a demo repository to show health checks (https://github.com/tomwillfixit/healthcheck).

Those examples are very basic, but can be useful in itself (possibly we may want to add a "real" example as well).

/cc @mstanleyjones

also /cc @dongluochen @tianon who may be able to provide more information on this subject 👍