CI: native arm64 runners, registry buildx cache, drop arm/v7

[rick-lam]

Summary

Three CI-speed-up changes bundled together, motivated by the post-merge build of #44 spending 30–60 min per PHP 8.3+ job under QEMU emulation:

• Native arm64 runners. Build phase is split per (version, platform). The linux/arm64 slices now run on ubuntu-24.04-arm (free for public repos) instead of going through QEMU on an amd64 runner. amd64 stays on ubuntu-latest. No platform requires QEMU anymore.
• Registry-backed buildx cache. Each per-arch build reads and writes a per-(version, platform) cache tag (graze/php-alpine:buildcache-VER-SLUG) so subsequent runs reuse layers.
• Drop linux/arm/v7 everywhere. 32-bit ARM has no common modern PHP deployment target and was the worst case under QEMU. Removing it from all versions (not just 8.3+) eliminates QEMU from the workflow entirely and shrinks the build matrix from a hypothetical 31 to 20 jobs.

Note on tag continuity: Existing published tags (e.g. graze/php-alpine:7.4) already include an arm/v7 manifest entry. After this PR merges, the next push to master republishes those tags as multi-arch manifests without arm/v7. Consumers pulling on a linux/arm/v7 host will get no matching manifest until they pin to an earlier digest.

How the workflow is restructured

• matrix job emits an expanded build matrix: each versions.json entry's platforms field is split into per-arch tuples with the right runner attached.
• test job is unchanged — single-arch amd64 build + bats, runs on PRs and pushes.
• build job runs per (version, platform), pushes by digest (no tag), uploads digest artifact. No QEMU step.
• New manifest job runs per version on master push only, downloads digests, and uses docker buildx imagetools create to publish multi-arch tags including the 5/7/8/latest/test aliases.

Notes

• Cache tags (buildcache-*) appear in Docker Hub alongside real tags; can be moved to a separate cache repo later if too noisy.
• First push after merge rebuilds without cache; speedup compounds from the second push onward.

Test plan

• PR build runs test for all 12 versions; deploy jobs are skipped on PR.
• After merge, build matrix produces 20 jobs; arm64 jobs land on ubuntu-24.04-arm.
• manifest job assembles correct tags incl. :5/:7/:8/:latest/:test aliases.
• docker manifest inspect graze/php-alpine:8.5 shows amd64 + arm64 only.
• docker manifest inspect graze/php-alpine:7.4 shows amd64 + arm64 only (no arm/v7).
• Wall-clock for 8.3/8.4/8.5 drops materially vs the pre-change run.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Speeds up the GitHub Actions build by (a) splitting the build job per (version, platform) so arm64 slices can run on native ubuntu-24.04-arm runners instead of QEMU, (b) adding a registry-backed buildx cache keyed per (version, platform), and (c) dropping linux/arm/v7 from PHP 8.3+. The original single multi-arch build step is replaced by a digest-only per-arch build followed by a new manifest job that assembles the final tags with docker buildx imagetools create.

Changes:

• Restructured build.yml into matrix → test → build (per arch, digest push) → manifest (per version, tag assembly), with arm64 on native runners and QEMU only for arm/v7.
• Added per-(version, platform) registry cache tags buildcache-VER-SLUG for both main and test images.
• Removed linux/arm/v7 from the versions.json entries for 8.3, 8.4, and 8.5.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
versions.json	Drops linux/arm/v7 from the 8.3, 8.4, 8.5 platform lists.
.github/workflows/build.yml	Splits build per arch onto native runners, adds registry cache, and adds a manifest job that publishes tags via imagetools create from uploaded digest artifacts.

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