Limit file descriptors to avoid Erlang CPU bug

The rabbitmq container has a high open file limit, which causes Erlang to use up unnecessarily large amounts of CPU in the child_setup process.

See: http://erlang.org/pipermail/erlang-questions/2013-June/074205.html
And:  https://groups.google.com/forum/#!msg/rabbitmq-users/hO06SB-QBqc

This patch fixes the issue by setting the open file limit to 1024 via an extra ulimit instruction. In theory it should also be possible to set this via `/etc/security/limits.conf`, but I was unable to get this to work under Docker.