Add native ARM64 gateway and runner images

[theonlypal]

Description

This PR completes #1760 by publishing native ARM64 gateway and runner images alongside AMD64 and assembling stable multi-platform manifests. Worker images and CLI release binaries already supported ARM64; this closes the remaining official-image gaps and removes the gateway's AMD64-only JuiceFS installation path.

Changes

• Build gateway release and dispatch images natively on AMD64 and ARM64.
• Publish versioned, latest, and custom-tag gateway manifests only after both architectures succeed.
• Add an ARM64-safe JuiceFS installation path while preserving the existing Beam-pinned AMD64 build.
• Build every Python and Micromamba runner variant natively on AMD64 and ARM64.
• Publish versioned, latest, and custom-tag runner multi-platform manifests.
• Keep Helm chart and BYOC publication behind successful dual-architecture gateway builds.
• Correct stale workflow job names and remove invalid legacy build-push inputs in the modified release paths.

Kubernetes can now resolve the correct gateway, worker, and runner image architecture from the unsuffixed multi-platform tags, including mixed-architecture clusters, without requiring architecture-specific Helm values.

Verification

• Actionlint passes all four modified workflows.
• Workflow topology assertions confirm both architecture matrices, build dependencies, and manifest coverage for all runner variants.
• Gateway, worker, and agent cross-compile for linux/amd64 and linux/arm64.
• The complete ARM64 gateway production image builds and runs JuiceFS 1.3.1 and mount-s3 1.5.0.
• The AMD64 gateway base image builds successfully and preserves the existing Beam-pinned JuiceFS path.
• All ten ARM64 runner variants build and execute with their expected Python versions.
• go test ./pkg/... passes.
• SDK suite passes with 201 tests and 2 intentional skips.
• git diff --check passes.

Closes #1760

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Summary by cubic

Adds native ARM64 builds for the gateway and all runner images, and publishes multi-platform manifests so Kubernetes pulls the correct architecture automatically. Also adds an ARM64-safe juicefs install and gates Helm/BYOC publish on dual-arch success. Closes #1760.

• New Features

  • Build gateway and all Python/Micromamba runners natively for linux/amd64 and linux/arm64.
  • Publish versioned, latest, and custom multi-platform manifests after both architectures succeed.
  • Add ARM64-safe juicefs install in docker/Dockerfile.gateway; keep the AMD64 Beam-pinned path.
  • Clean up workflows: corrected job names and removed invalid legacy inputs.

• Migration

  • No arch-specific Helm values needed; use unsuffixed tags and Kubernetes selects the right image.
  • Mixed-architecture clusters work without changes.

^{Written for commit 335ad4e. Summary will update on new commits.}