Bump alpine to 3.23, add platforms to docker-dryrun

[silverwind]

Next attempt for alpine 3.23 following #36185 and #36202.

Do not merge yet, I want to try if we can get this build to execute during docker-dryrun CI so that such main branch failures can be avoided in the future.

[silverwind]

I've now added the platforms to docker-dryun, same es in the release build, so these two docker builds should now do the exact same builds.

gitea/.github/workflows/release-nightly.yml

Lines 115 to 122 in 12a81d3

	- name: build regular docker image
	uses: docker/build-push-action@v6
	with:
	context: .
	platforms: linux/amd64,linux/arm64,linux/riscv64
	push: true
	tags: ${{ steps.meta.outputs.tags }}
	annotations: ${{ steps.meta.outputs.annotations }}

[silverwind]

BTW docker-dryrun is quite slow after this change (99minutes), but it's definitely necessary.

I think I can refactor the actions code that it only runs when Dockerfiles are modified, keeping the slowness impact to a minimum.