feat: add linux/arm64 multi-arch image support

[gbartolini]

Introduces linux/arm64 support alongside the existing linux/amd64 images.

• Dockerfile: drop the hardcoded --build=x86_64-linux-gnu configure flag (let it auto-detect) and compute --libdir from dpkg-architecture -qDEB_HOST_MULTIARCH instead of hardcoding the amd64 multiarch triplet.
• docker-bake.hcl: turn the target's platforms list into an overridable variable, defaulting to both linux/amd64 and linux/arm64.
• .github/workflows/reusable-build.yml (new): builds each architecture natively in its own matrix job (amd64 on the existing runner, arm64 on ubuntu-24.04-arm) and merges the two per-arch images into a single multi-arch manifest per tag via docker buildx imagetools create. PostgreSQL is compiled from source, which is CPU-bound, so this avoids cross-building arm64 through QEMU emulation on an amd64 runner, which would multiply build time considerably.
• build.yml, build-commitfest.yml, continuous-delivery.yml: extract the existing input/tag-resolution steps into a prepare job and delegate the actual build to the new reusable workflow. On pull_request runs, continuous-delivery.yml now builds both architectures to validate the Dockerfile without pushing or publishing a manifest (previously it built only amd64).
• README.md: document the multi-arch build/merge approach and how local docker buildx bake builds are affected (QEMU emulation for the non-native platform).

Closes #151

Assisted-by: Claude