Hi @Shengjie_Xu,
What you are reporting is a consequence of the defaults when you install Docker (i.e., it doesn’t have to do with fmriprep or fmriprep-docker). Please have a look to their documentation if you want to be able to run docker as a non-root user (which is highly recommended).
Incidentally, following up the guide will allow you to use the fmriprep-docker command.