removed usage of variable docker image name in build.gradle
removed usage of variable docker image name in build.gradle

mesosphere/logstash,kensipe/logstash,triforkse/logstash-mesos,drewrobb/logstash,drewrobb/logstash,kensipe/logstash,frankscholten/logstash,mesosphere/logstash,triforkse/logstash-mesos,drewrobb/logstash,frankscholten/logstash,mesosphere/logstash,alex-glv/logstash,triforkse/logstash-mesos,suppandi/logstash,mesos/logstash,kensipe/logstash,alex-glv/logstash,drewrobb/logstash,frankscholten/logstash,suppandi/logstash,suppandi/logstash,suppandi/logstash,triforkse/logstash-mesos,alex-glv/logstash,frankscholten/logstash,mesosphere/logstash,alex-glv/logstash,kensipe/logstash,mesos/logstash