feat: build, test, and publish multi-arch (amd64 + arm64) images

[austinderrick]

Refs #424

This publishes solidnerd/bookstack for linux/amd64 and linux/arm64, and runs the e2e test on both architectures before anything is pushed.

The earlier arm64 attempt (#321) was reverted because the e2e test could not run on ARM. GitHub now offers native ubuntu-24.04-arm runners for public repositories, so each architecture is built and tested on its own runner, the approach @rjhenry asked for in #424:

• build and e2e run as a matrix over amd64 (ubuntu-24.04) and arm64 (ubuntu-24.04-arm). Each arch uploads its own image-bookstack-<arch> tarball, and e2e loads it and runs make e2e natively.
• master and release now run make e2e too, so nothing is pushed unless both architectures pass.
• push builds linux/amd64,linux/arm64 and reads the layers the build jobs wrote to the GitHub Actions cache (type=gha, one scope per arch), so the arm64 layers come from the native build instead of a rebuild under QEMU. The unused artifact download and docker load in the push job are removed.
• The e2e test container changes from appropriate/curl, which is amd64-only, to curlimages/curl:8.23.0, which publishes arm64. The other images involved (php:8.5-apache-trixie, alpine:3, mysql:26.7) already publish arm64.

Tested locally on an arm64 Mac: make build followed by make e2e builds an arm64 image, and BookStack serves /login to the curl check.