See this section in the README
Freesurfer 6 will remain the default image till 2024, at which point Freesurfer 7 will become the default.
You can get the default version with docker pull bids/freesurfer.
Freesurfer 7 is available at docker pull bids/freesurfer:7.
Freesurfer 6 is available at docker pull bids/freesurfer:6.
See this section in the README