These images are built automatically by hub.docker.com
A base image to build RapidPro on top of, installs heavier libraries like geos and gdal so each individual build doesn't have to.
Originally this was just tagged as :latest and built it for v3. With RapidPro v4 being released this was no longer adequate.
For RapidPro v3 builds use: rapidpro/rapidpro-base:v3
For RapidPro v4 builds use: rapidpro/rapidpro-base:v4
rapidpro-base:latest still points at v3 for backwards compatibility but consider it deprecated.