Add documentation for new postgres initdb logic

[tianon]

Documentation change for docker-library/postgres#75.

[yosifkit]

LGTM

[md5]

It might be good to point out that psql commands should generally be run as user postgres and that no password is necessary owing to the use of trust authentication for a local Unix socket connection.

[tianon]

@md5 agreed; want to open an issue or PR? 😇

[md5]

I could do that. However, I think that the issue of whether *.sql files should be run as postgres or $POSTGRES_USER should probably be resolved first, since both of those could probably be documented at the same time.

[md5]

cf. docker-library/postgres#81

[tianon]

👍

[md5]

See docker-library/postgres#82 for the entrypoint update and #319 for the docs update.