When you are looking for a secure Docker base image, what are you looking for excactly?

At Flownative, we run a lot of tools and applications in Docker containers – most prominently Flownative Beach. Early on, we decided to not rely on off-the-shelf images for PHP, Redis and all the other components we need, because we wanted to know what we are running and not just hope that image maintainers will keep an eye on security issues.