Create container image for Phlex development

[knoepfel]

The container image should include:

• boost
• catch2
• cmake
• fmt
• gcc@15
• intel-tbb
• jsonnet
• root
• spdlog

[knoepfel]

Package available at https://github.com/orgs/Framework-R-D/packages/container/package/phlex-ci.
Dockerfile and entrypoint script available at https://github.com/Framework-R-D/phlex/tree/main/ci.