Provide an offical docker image

[zhengxinchang]

Hi, developers,

Thank you for developing this excellent tool! Is it possible to provide an official Docker image for users? That would be dramatically helpful for those working on clusters without root permissions.

Sincerely,
Xinchang

[colindaven]

There is a singularity image listed in the README.md and a singularity def file in the repo.

I got it here :

#46