Pull Alpine/aarch64 tarball from unofficial builds as we do for Alpine/x64

Although unoffical-builds does produce an arm64 binary, it is not used by the images here. I think mixed among the recent-ish discussions was whether we should just drop even that pre-built, and just use the Docker infrastructure for all the Alpine variants.

While there are reasons that i won't go into here why that might be desirable, IMHO that would push it further away from the "regular" tier1/2 platforms. Since we're building it anyway (and people are downloading it independently of the docker images at a rate of about 1000/hour) I'd prefer the container image to have the identical build that we produce in the unofficial projects.

I'll spin off another issue about whether to process the unofficial Alpine/aarch64 built in the same way as Alpine/x64 since that's separate from Alpine/s390x.

Originally posted by @sxa in https://github.com/nodejs/docker-node/issues/2462#issuecomment-4287416228

The Alpine/aarch64 builds are far less popular than the figure listed above but since https://github.com/nodejs/unofficial-builds/pull/189 got merged we do now now have cross-compiled builds of Alpine/arm64 in the unofficilal-builds alone with x64 so we could follow the same process for both.

Notes:

• It's cross compiled instead of build natively (May not be an option, but we don't have any cross-compiled builds yet which have the rust support working for temporal enablement yet ... But that's potentially a bigger problem). It's also a cross-compiled build which doesn't undergo any testing elsewhere in the node system. https://github.com/nodejs/unofficial-builds/issues/200 is a potential issue with the cross-compilated build (SEA failure)
• There is a proposal to bump Alpine up to tier 2 which would change things again
• With the removal of Alpine/s390x from node 26's container images, and Alpine/ppc64le having been removed some time ago, that only leaves Alpine/arm64 as the "dynamically built at image creation time" platform so this would ultimately help reduce complexity in the process.

Beginne mit dem Docker-Image-Build-Prozess und vergleiche, wie Alpine/x64 aus unofficial-builds abruft. Lies unofficial-builds PR #189 und Issue #200, einschließlich der Hinweise zur Cross-Kompilierung und zum SEA-Fehler. Als abgeschlossen gilt, wenn Alpine/arm64 konsistent mit Alpine/x64 verarbeitet wird, ohne die genannten Build- und Testaspekte zu übersehen.