Slack notification within docker.

[Evedel]

Hello! First of all thank you for the great app! It is not actually the issue, but as far as I saw questions about it some times, may be it will be worth to know for you also. The common part is I can send test notification for slack, but it does not work for the real, triggered issues. As I mention early, I use docker, and obviously, I installed sentry-slack only inside of container with webfront app, and this is why I can send it with test button. And, obviously, this is why other container with sentry worker could not sent any messages, it is just does not have any installed sentry-slack plugin. Maybe it will be helpful to stress this info somewhere.