Add support for heartbeat parameter

[peterstory]

The heartbeat parameter should be configurable via an environment variable. I made it configurable with just a couple tweaks to the entrypoint.
https://www.rabbitmq.com/heartbeats.html

[yosifkit]

Especially with 3.7's simpler config format, we'd rather move to doing less config via the entrypoint. Since it will be simpler for a user to supply their own config file and then the entrypoint can sed their file to update/add a limited set of specific values rather than being responsible for every option possible in a rabbit config.

You should be able to add any config option with RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS as noted in docker-library/docs#1011.

Related open issues asking for extra config options: #44, #76, #79, #85, #110, #116, #118, #164, #185, #186.

[peterstory]

Ah, very good. Thanks for pointing out RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS.

[tianon]

👍 ❤️