Use podman to build with unprivileged user

Being able to use Docker from an unprivileged user (as documented in the FAQ) can lead to privilege escalation. This is security issue, which can be avoided by using other tools which are more suited for "unprivileged user" use-case, such as podman.

This Merge Request modifies Makefile in order to add a from_podman target, which uses podman to launch a container instead of docker.

Moreover use fully-qualified image names instead of short names, in order to follow good practices documented on https://www.redhat.com/en/blog/be-careful-when-pulling-images-short-name.