Add legacy glibc-compatible Linux build path

[tctco]

Motivation

• Provide a reproducible Linux build path that targets older glibc baselines so produced binaries run on older images that may lack glibc 2.34.
• Avoid accidentally shipping Linux packages that reference symbol versions newer than GLIBC_2.17 by building on a CentOS7/manylinux2014 toolchain.
• Keep legacy dependency/build caches separate from the default Ubuntu development container to prevent cross-contamination of binaries and Conan packages.

Description

• Add docker/Dockerfile.core.legacy based on quay.io/pypa/manylinux2014_x86_64 and devtoolset-10 to target a glibc 2.17 compatibility baseline and install required build tooling and Python packages.
• Add a dccc-core-legacy service in docker-compose.core.yml with separate Conan/CMake volumes and CONAN_CPPSTD=gnu17 to run legacy builds without mixing caches with the default container.
• Modify .github/workflows/cross-compile.yml to build and run the legacy Docker image for Linux jobs and add a post-build GLIBC symbol-version verification step that fails the job if any packaged ELF references symbols newer than GLIBC_2.17.
• Update localizer/src/README.md to document how to build using the legacy container and explain the compatibility intent and separate volumes.

Testing

• Ran bash -n scripts/docker-build-core.sh to validate the build script syntax and it succeeded.
• Parsed and validated YAML files with ruby -e 'require "yaml"; YAML.load_file(".github/workflows/cross-compile.yml"); YAML.load_file("docker-compose.core.yml")' and it succeeded.
• Verified the legacy Dockerfile contains expected markers with a small python3 check for manylinux2014_x86_64, devtoolset-10, and CONAN_HOME and it succeeded.
• Attempting to run the Docker/compose build could not be executed locally because docker is not installed in the current environment, so full image builds and end-to-end CI validation will run in CI where Docker is available.

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Codex Task