Housekeeping – add .dockerignore, add INFO level logging, slim dependencies in Dockerfile

[secondfry]

1. Add .dockerignore with /mongodb as the only line, so one won't transfer entire MongoDB as context to bot Python container.
2. Add INFO level logging with logging.basicConfig(level=logging.INFO) so user won't question themselves if bot is running or not.
3. Slim dependencies in Dockerfile by installing only necessary Ubuntu packages and lower build time by rearranging Dockerfile instructions – now it caches pip packages install step properly.

[secondfry]

Closing this as I've started messing with configs.
Opened another one without those changes – #304