Create custom Docker build image for Linux x64 (ubuntu:20.04 + build tools)

[Blind-Striker]

Context

The Linux x64 build uses an ubuntu:20.04 container to target glibc 2.31 for maximum binary compatibility. This is the correct strategy (used by SkiaSharp, LibGit2Sharp, .NET Runtime), but stock ubuntu:20.04 is increasingly behind on build-time tooling:

• autoconf 2.69 — gperf 3.3 (via harfbuzz) requires >= 2.70, forcing source install in CI
• python3-venv missing — libsystemd's meson build needs it, requires explicit apt install
• Each vcpkg baseline bump risks more build-time tool gaps

Currently these are patched inline in prepare-native-assets-linux.yml, but this is fragile and adds CI time on every run.

Goal

Create a custom Docker image for Linux x64 builds that:

1. Starts from ubuntu:20.04 (preserving glibc 2.31 target)
2. Pre-installs all build prerequisites (autoconf 2.72, python3-venv, X11/Wayland/ALSA dev packages, etc.)
3. Is published to GitHub Container Registry (ghcr.io) with version tags
4. Replaces the inline apt-get install + autoconf source build steps in the Linux workflow

Industry Reference

• SkiaSharp: Uses custom Docker images with dockerbuild.sh for controlled glibc builds
• LibGit2Sharp: dockerbuild.sh with pinned container environments
• .NET Runtime: Builds on oldest supported distro for glibc floor

Deliverables

1. Dockerfile for linux-x64 build image (ubuntu:20.04 based)
2. GitHub Actions workflow to build and push the image to ghcr.io
3. Update prepare-native-assets-linux.yml to use the custom image for x64
4. Document the image build/update process in docs/playbook/

Exit Criteria

• Custom image published to ghcr.io with version tag
• Linux x64 CI job uses custom image instead of stock ubuntu:20.04
• Build prerequisites step in workflow is minimal (no more source installs)
• Image build/update workflow documented
• glibc 2.31 target preserved