CI: Pre-bake curl, ca-certificates, and python3 into smoke build Dockerfiles

Include curl, ca-certificates, and python3 in the slim smoke test build container images (Ubuntu 24.04 amd64, Ubuntu 24.04 arm64, and Ubuntu 26.04 riscv64 cross-compile).

This ensures containers running with EIGEN_CI_SKIP_APT=true can download compiler caching tools and interact with authentication endpoints without invoking apt-get during CI jobs.
3 files changed