Add multi-architecture GitHub Actions workflow for PHP extension builds

[Copilot]

Adds automated builds for Linux x86_64 and arm64 across PHP 8.1-8.4, supporting both glibc (Debian) and musl (Alpine) variants.

Changes

• Dockerfile: Multi-stage build copying Rust 1.92 toolchain from official image into PHP base images. Configurable via PHP_VERSION and BASE_IMAGE build args.

• GitHub Actions workflow (.github/workflows/build-multi-arch.yml):

  • Matrix strategy covering 16 build combinations (4 PHP versions × 2 architectures × 2 libc types)
  • Uses Docker Buildx with QEMU for cross-compilation
  • Uploads artifacts named libext_shopware-php{VERSION}-{ARCH}-{LIBC}.so
  • Includes smoke tests to verify extension loads

• Local build script (build-local.sh): Wrapper for Docker builds matching CI configuration

• Documentation:

  • BUILD.md: Build instructions and troubleshooting
  • .github/ARTIFACTS.md: Artifact download and installation guide
  • Updated README.md with pre-built binaries section

Usage

Local builds:

./build-local.sh 8.4 bookworm x86_64
./build-local.sh 8.3 alpine3.20 arm64

CI artifacts available at Actions → Build Multi-Architecture → Artifacts

Build Matrix

PHP	x86_64	arm64	Variants
8.1-8.4	✓	✓	glibc (Debian Bookworm), musl (Alpine 3.20)

Original prompt

add GitHub Workflow which builds it for Linux x86 arm64 and for various PHP versions. also for musl and gnu Linux. we should use an docker file so we can test it also locally

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