[ecbadeaux]

Summary

Fixes two ways the local build breaks. No C++ changes.

• build.sh fails on macOS. It writes compiler.version=15.2 into the default Conan profile on every OS. On macOS that profile describes Apple clang, so conan install stops with ERROR: Invalid setting '15.2' is not a valid 'settings.compiler.version' value. The script now sets the compiler version only on Linux. On macOS, conan profile detect already records the installed Apple clang version.
• The Docker image fails to build when the checkout has a host venv/. The venv is copied into the image with its interpreter links still pointing at host paths (on macOS, Homebrew's Python), so setup-venv.sh fails with No such file or directory: '/home/ubuntu/spectator-cpp/venv/bin/python3'. The image no longer copies venv/ or any cmake-build* directory.

Changes

• build.sh: sets compiler.cppstd=23 on every OS, as before, and compiler.version=15 on Linux only. Conan's profile detection records gcc 5 and later by major version, and build.sh exports CC=gcc-15 before detection, so 15 is what a fresh Linux profile already contains; the edit only corrects older profiles. On Linux the only change is 15.2 → 15. Both are valid for gcc, and build.sh builds every dependency from source either way, so CI is unaffected.
• Dockerfiles/Ubuntu.Dockerfile: installs gcc-15 and g++-15 instead of gcc-13 and g++-13, adding CI's toolchain PPA and the other packages CI installs (binutils-dev, libiberty-dev). build.sh requires gcc-15. The image currently gets it only because ubuntu:latest is 26.04, whose build-essential defaults to gcc-15; on a 24.04 base, build.sh stops with gcc-15 is required but not found.
• Dockerfiles/Ubuntu.Dockerfile.dockerignore: excludes cmake-build*/ and venv/, instead of only cmake-build/.
• .gitignore: cmake-build-*/ replaces the separate debug and release entries, so every cmake-build-* directory stays untracked.

Testing

• Linux: built the image from this branch (Ubuntu 26.04.1, gcc 15.2.0) and ran ./build.sh in it, with Conan's build jobs capped at 4 to fit an 8 GB Docker VM. The profile got compiler.version=15 and compiler.cppstd=23, every dependency built from source, and all 18 tests passed.
• macOS (Apple clang 21): ./build.sh left the profile's compiler.version=21 alone, set compiler.cppstd=23, built every dependency from source, and all 18 tests passed.
• Docker context: with a macOS venv/ in the checkout, main's image fails at setup-venv.sh (exit code 127). With a macOS venv/ and a cmake-build-release/ in the checkout, this branch's image builds, its venv uses the image's Python, conan runs, and the build directory isn't copied.
• Conan 2.29: rejects compiler.version=15.2 for apple-clang, and accepts both 15 and 15.2 for gcc.

Note for macOS

If you've run build.sh from main, ~/.conan2/profiles/default already says compiler.version=15.2, and this change no longer touches that line. Run conan profile detect --force once to restore it; build.sh re-applies compiler.cppstd=23.