[DOCKER] latest tag ambiguity

[mgxd]

Seeing as how we plan on supporting two series at once (LTS, 21/22), we may need to rethink the latest tag, since it currently just points to the last release (for example, latest could either be 20.2.7 or 21.0.0).

Two options come to mind:

• use latest for the latest series only (remove latest docker push to from maint/ branches)
• use the series prefix, i.e. latest-20.2.x

[effigies]

I would remove it from the older maint/ branches once a new series is released. We discourage people from using it; updating it is mostly to keep people from running docker pull nipreps/fmriprep and getting an extremely old version.