Response time problem for rails app in docker

OS: ubuntu 16.04LTS
Docker version 17.05.0-ce, build 89658be
steps to reproduce: not available

I have a rails web-app running under puma in docker.
On two developer hosts we have a problem with the response times of the web-app.
When I call the app from outside docker I observe in the rails-logs an immediate response.
The request is completed very fast. However curl or the webbrowser waits up to 30 seconds for the request.
This time seems to get lost somehow between the completed web-app inside docker and the outside docker-world.
When I run the same web-app not in docker it works smooth without response-time problems.

I think it must be somehow a “network”/“dns” problem. But I have not really a plan how to investigate the problem
further. The problem does not appear on other hosts with the same OS and docker version.
Waht steps should I do to get closer to the problem?

/dev/random was the problem. When I map the volume
into the container it works.