Setting Graylog uid/gid as arguments in the docker build instead of being hard coded in the Dockerfile.

Hello, we recently had to modify the graylog Dockerfile to allow set the graylog uid/gid since we are using the application with LDAP.

I have modified the Dockerfile and the build to adhere to this and a modified docker-compose. I would like to contribute these changes in a branch for the merge is possible.

Thanks
Conrad