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.

Start with the Docker image build process and compare how Alpine/x64 pulls from unofficial-builds. Read unofficial-builds PR #189 and issue #200, including the notes about cross-compilation and SEA failure. Done means Alpine/arm64 is processed consistently with Alpine/x64 without overlooking the stated build and testing concerns.