GH Actions: fix issues with CI

[romeroalx]

Included in this PR are fixes for:

• Version of docker-py set to 7.1.0. Older versions require a specific version of requests to work (see here
• Move from tests fromcentos:7 to oraclelinux:7
• Adapt tests to comply with newer versions of auth that show a different string for pdns_server --version and also through stdout instead of stderr.
• Skip SSL validation for the mysql client in archlinux
• Mount /sys/fs/cgroups as rw to overcome the error that made containers fail after starting: Failed to create /actions_job/b23bdd0759a907717ae61975e6bf5b5044ae8f9dee961e93a4c108e30868f395/init.scope control group: Read-only file system

[SrX]

LGTM