Allow users to use RABBITMQ_ERLANG_COOKIE_FILE to specify the path to an Erlang cookie file (especially for Docker secrets)

[tianon]

Closes #279

[yosifkit]

This seems like duplicate work. We put this value in the default rabbitmq cookie file:

rabbitmq/3.7/docker-entrypoint.sh

Lines 164 to 176 in 406d5ee

	if [ "${RABBITMQ_ERLANG_COOKIE:-}" ]; then
	cookieFile='/var/lib/rabbitmq/.erlang.cookie'
	if [ -e "$cookieFile" ]; then
	if [ "$(cat "$cookieFile" 2>/dev/null)" != "$RABBITMQ_ERLANG_COOKIE" ]; then
	echo >&2
	echo >&2 "warning: $cookieFile contents do not match RABBITMQ_ERLANG_COOKIE"
	echo >&2
	fi
	else
	echo "$RABBITMQ_ERLANG_COOKIE" > "$cookieFile"
	fi
	chmod 600 "$cookieFile"
	fi

I think we just need to document where to mount the cookie file like the Dockerfile alludes to:

rabbitmq/3.7/debian/Dockerfile

Lines 128 to 129 in 406d5ee

	# set home so that any `--user` knows where to put the erlang cookie
	ENV HOME /var/lib/rabbitmq

[tianon]

Ah, good point -- I forget that Docker secrets can be arbitrary mounted anywhere. I'll make a docs PR instead. 👍

Edit: docker-library/docs#1312