CI: add release multi-arch workflow for git-bridge image

[Musicminion]

Motivation

• Automate building and publishing multi-arch release images for the git-bridge service so release pipelines produce both linux/amd64 and linux/arm64 artifacts.
• Reuse the existing release workflow pattern (per-arch build -> upload digest -> create manifest) to keep behavior consistent with other images.

Description

• Added two jobs to .github/workflows/build_release_image.yml: build_git_bridge to build and upload per-architecture images from services/git-bridge/Dockerfile and merge_git_bridge to create and push a multi-arch manifest for ghcr.io/.../git-bridge with latest and release-version tags.
• build_git_bridge runs for linux/amd64 and linux/arm64, extracts the latest release-v* ref, sets version, runs docker/build-push-action@v6 with context: ./services/git-bridge, and uploads digest artifacts per-arch.
• merge_git_bridge downloads the per-arch digest artifacts and uses docker buildx imagetools create to assemble and push the manifest, then runs an imagetools inspect and retention cleanup for overleaf-pro/git-bridge.
• The services/git-bridge/Dockerfile was inspected and left unchanged, and the workflow reuses metadata labels and registry login consistent with the existing release jobs.

Testing

• Parsed the updated workflow YAML with Ruby using ruby -e "require 'yaml'; YAML.load_file('.github/workflows/build_release_image.yml'); puts 'ok'", which returned ok indicating valid YAML.
• Attempted to validate with Python YAML but the environment lacked PyYAML causing the ModuleNotFoundError, which is an environment issue and not a workflow error.
• Committed the changes and recorded the change with a local commit (ci: add multi-arch release workflow for git-bridge image).

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