[docker] Scanning for images that contain name "beat" on docker images is not restrictive enough

Currently, this package tests for all docker images with the string beat on its name. This potentially includes anything produced inside the beat gitlab group. Currently, we have an image that is produced to serve as the basis for compiling another project and 5 version of it are installed on the docker daemon. This results in probing a bunch of useless images and waste time on the CI. As a consequence, many tests are failing.

I propose we just restrict these to images containing the substring beat.env. instead.