feat(docker): add multi-arch builds for amd64 and arm64

[alexkreidler]

Summary

• Build and push multi-platform Docker images (linux/amd64 + linux/arm64) so images work natively on ARM hosts (e.g. Apple Silicon Macs)
• Local builds stay single-platform due to buildx type=docker limitation; multi-arch only applies when pushing to registry
• Remove hardcoded platform: linux/amd64 from docker-compose.yaml

Closes #5576 and closes NAN-4926

Changes

• scripts/build_docker.sh — push action now builds linux/amd64,linux/arm64 via type=registry
• scripts/build_docker_self_hosted.sh — restructured to build+push multi-arch in one step when PUSH=true
• .github/workflows/build-image.yaml — split build step so authenticated builds use push (multi-arch)
• docker-compose.yaml — removed platform: linux/amd64 from nango-server

Test plan

• Local build action succeeds (amd64, type=docker)
• Self-hosted local build succeeds
• CI builds both platforms and pushes multi-arch manifest to Docker Hub

🤖 Generated with Claude Code

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This PR updates Docker build scripts and CI to produce multi-architecture images for linux/amd64 and linux/arm64 when pushing to a registry, while keeping local builds single-arch due to buildx output constraints. It also removes the hardcoded platform override from docker-compose.yaml so the hosted image can run natively on ARM hosts.

Possible Issues

• If PUSH is not passed as true, scripts/build_docker_self_hosted.sh will default to amd64-only builds, which may surprise callers expecting multi-arch.
• CI multi-arch builds require buildx and registry auth; failures will now block multi-arch push paths.

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This summary was automatically generated by @propel-code-bot

[propel-code-bot]

No issues found; multi-arch build updates look sound and ready.

Status: No Issues Found | Risk: Low

Review Details

📁 4 files reviewed | 💬 0 comments

Instruction Files

├── .claude/
│   ├── agents/
│   │   └── nango-docs-migrator.md
│   └── skills/
│       ├── agent-builder-skill/
│       │   ├── EXAMPLES.md
│       │   └── SKILL.md
│       ├── creating-integration-docs/
│       │   └── SKILL.md
│       └── creating-skills-skill/
│           └── SKILL.md
├── AGENTS.md
└── GEMINI.md