doc: add list of images in README.md

README.md

 Dockerfiles to use for cubicweb.
+
+Images:
+
+```
+${CI_REGISTRY}/cubicweb/dockerfiles/buster-slim-pg11
+${CI_REGISTRY}/cubicweb/dockerfiles/kubectl-kustomize
+${CI_REGISTRY}/cubicweb/dockerfiles/kubectl
+${CI_REGISTRY}/cubicweb/dockerfiles/python-logilab:latest
+```