Mixed bags of fixes and polish

• gitlab-ci: remove unused debug (.pre) stage
• gitlab-ci: install devtools without its dependencies
• gitlab-ci: de-duplicate matrix option
• README: s/docker/podman/ in the requirements
• Drop erroneous oci- prefix for image-base{,-devel} targets
• README: rewrap, use standalone references
• Use the in-tree mirrorlist

Here's a bunch of changes that I've accumulated recently. All should be dead-obvious, although if you have any questions do let me know.

Note: the CI is failing although that seems to be unrelated to my changes. According to the pipelines page it started failing about 5 days ago - https://gitlab.archlinux.org/archlinux/archlinux-docker/-/pipelines?page=1&scope=all

@hashworks