Logstash 1.5.0-RC3 with added gemfiles. Observe that a local Gemfile. It is https://github.com/RallySoftware/logstash-output-sqs added manually, have a look in the Gemfile.
docker build -t logstash .
docker run -v /path/to/logstash.conf:/tmp/logstash.conf logstash -f /tmp/logstash.conf
Add your plugin to Gemfile and run bundle install. Then checkin Gemfile and Gemfile.lock.
Automated build on https://registry.hub.docker.com/u/regius/docker-logstash