The gitbucket service should be executed by non-root user.

[kanekoh]

If the service doesn't require privileges, it is a good way to use USER to change to non-root user(1).
That makes easy to run on Kubernetes and OpenShift(2).

(1) https://docs.docker.com/develop/develop-images/dockerfile_best-practices/
(2) https://docs.okd.io/latest/creating_images/guidelines.html#openshift-specific-guidelines