Run docker server on a fresh folder:
docker -d -g /home/core/docker-build/ -p /var/run/docker-build.pid -H tcp://127.0.0.1:5011
In the gnatsd-dockerfile project:
docker -H localhost:5011 build -t localhost:5011/gnatsd .
Now, the docker folder can be converted to a tarball:
$ time tar cvfz docker-gnatsd.tgz docker-build/
...
real 1m50.452s
user 0m48.342s
sys 0m13.691s