setup.py doesn't appear include redis as a dependency. It also didn't appear to be included in the requirements.txt file.

https://github.com/mher/flower/blob/master/setup.py#L51
https://github.com/mher/flower/blob/master/requirements.txt

I use redis as a broker and found that I needed to include redis in the pip install step to get this to work. Redis imported here:

https://github.com/mher/flower/blob/89987594e0fc3e8b99d012e9cac43d81493578c4/flower/utils/broker.py#L20

It might make sense to include installation of all the potential brokers in the image so that it works with all the brokers flower supports. Does this sound reasonable?