ci: Target RHEL8 and C++17

This MR:

• Uses container image based on Ubi8 (the minimal RHEL8 available to our docker images).
• Bumps SCDAQ compiler standard to C++17.
• Tweaks FindBoost.cmake to find version boost-devel version 1.78, which is available in the EPEL 8 package repository for our image.
• Fixes issue #37 (closed)

Depends on: merging the RHEL-based scdaq-builder

Upon approval: update the container registry URI in the build script.