Replace centos7 with rockylinux

Centos7 docker images are currently deprecated. On top of that the python version shipped with centos7 is 3.6 which is end of life. The natural successor seems to be rockylinux. In this commit I switched the distribution to see if at the very least the tests pass. It is still needed to check if the gfal2 python bindings preserved the same interface and behaviour.