Make tryghost/activitypub-migrations Dockerfile multiplatform

[touzoku]

• Pin sha256 of the image index, not the platform manifest digest. Use fully qualified container name.
• Use TARGETARCH to download the correct golang migrate binary

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 5aacf585-e407-47d7-ba7b-f8ca9b7e831b

📥 Commits

Reviewing files that changed from the base of the PR and between 4a14d50 and ff78ab0.

📒 Files selected for processing (1)

• migrate/Dockerfile

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Walkthrough

The migrate/Dockerfile is updated to enable multi-architecture Docker builds. The base Debian image reference is changed to the fully qualified docker.io/library/debian path (retaining the same digest). The migrate binary download URL is parameterized using the TARGETARCH build argument, switching from a hard-coded migrate.linux-amd64 artifact to migrate.linux-${TARGETARCH}, allowing the same Dockerfile to build for different CPU architectures. The remaining instructions (installing curl, copying bin and migrations directories, setting MYSQL_DB environment variable, and default CMD) remain unchanged.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: making the Dockerfile multiplatform-compatible, which is the primary objective of this PR.
Description check	✅ Passed	The description directly relates to the changeset, explaining the two key modifications: pinning the image sha256 with fully qualified container name and using TARGETARCH for the golang-migrate binary.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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