Use Fedora-41 based base image for all images

[lbarcziova]

With #622, we started building the base image with Fedora 41 as a base (instead of CentOS). We did the switch of using it in packit (packit/packit#2488) and packit-service (packit/packit-service#2673). There are still some repos building images based on CentOS, so we should do the switch there as well:

• dashboard - ci: switch to Fedora-based image dashboard#508
• specfile - ci: switch local Containerfile.tests to Fedora specfile#455
• sandcastle - ci: switch to Fedora-based image sandcastle#213
• fedmsg - ci: switch to Fedora-based image packit-service-fedmsg#117

Follow-up fixes

• ci: fix syntax in Dockerfile packit-service-fedmsg#118
• Fix broken image build from #213 sandcastle#214

[majamassarini]

@mfocko suggest to give higher priority to sandcastle. And probably ignore for now fedmsg dashboard.