resolves #282: add default healthcheck to dockerfiles

[zaquestion]

I didn't see an approach using pg_isready when browsing through #282 and other proposed changes, but perhaps someone has already been down this route?

Probably someone will have input on the actual HEALTHCHECK settings to use (e.g. interval, timeout, retries) I can make any requested changes real quick (provided some intentional reason ;)

P.S. The readme told me to look over at https://hub.docker.com/_/postgres?tab=description info on contributing, I might be tired and completely overlooking it, but I didn't actually see anything. Hopefully I've set this up correctly, LMK if theres anything at all desired from a contributor perspective.

[zaquestion]

The postgres docs (and some 3rd party release notes) seem to imply that this is supported since 9.3, but the docs them self also list 9.3 as incompatible, nevertheless everything in this repos seems to be >=9.4 so seemingly we're good there.

Ref: https://www.postgresql.org/docs/9.3/app-pg-isready.html

[zaquestion]

Closing per docker-library HEALTHCHECK policy

See: #282 (comment)

Ref: https://github.com/docker-library/faq#healthcheck