feat: add PHP 8.5 support and improve CI builds (#81)

* fix(renovate): pin PHP base images to digest-only updates

* feat(php85): add PHP 8.5 with Imagick and Composer

* ci: add php85 to build and PR matrices

* docs: update supported PHP version range to 8.5

* ci: enable GHA build cache for Docker image builds

* fix: address CR feedback — fix DOCKERFILE_PATH, renovate match, composer verify, drop python3-dev

* ci: use native arm64 runners for arm64 Docker builds

* ci: use build-by-digest with manifest merge for multi-arch images

Switch from direct per-arch push to a two-phase workflow:
1. Build phase pushes images by digest and uploads artifacts
2. Merge phase creates multi-arch manifest lists per PHP version

This ensures proper multi-arch manifest tags instead of
last-writer-wins race conditions between arch builds.

* fix: remove continue-on-error and suppress SC2046 shellcheck warning

Remove continue-on-error from build job so failed arch builds correctly
block the merge job from pushing incomplete manifests. Add shellcheck
disable directive for intentional word-splitting in manifest creation.

.github/renovate.json

@@ -1,4 +1,13 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
-  "extends": ["github>ivuorinen/renovate-config"]
+  "extends": ["github>ivuorinen/renovate-config"],
+  "packageRules": [
+    {
+      "description": "Only allow digest updates for PHP base images, not version upgrades",
+      "matchDatasources": ["docker"],
+      "matchPackageNames": ["php", "library/php"],
+      "matchUpdateTypes": ["major", "minor", "patch"],
+      "enabled": false
+    }
+  ]
 }