docker : prebuild web UI for s390x build

[aldehir]

Overview

ref: #24794 (comment)

The s390x docker build fails because the vite/svelte build uses lightningcss which does not have s390x binaries. This PR adds the ui-build workflow and uses it only in the s390x build. The other Dockerfiles are kept as-is, so users may build them directly to produce complete images.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes

[aldehir]

Struggling to find a good way to test s390x in my own repo. It seems IBM no longer allows new instances on their cloud platform:

Notice for s390x images

Images for the Virtual Server for VPC (s390x architecture) are deprecated. Starting 28 February 2026, you can't create new instances from s390x images. Existing instances are supported until 20 February 2027. Any instances that remain after that date are deleted. For more information and details on migration, see the deprecation announcement.

I may need to run a self-hosted runner on QEMU w/ software emulation. That sounds like it would crawl.

[CISC]

Struggling to find a good way to test s390x in my own repo. It seems IBM no longer allows new instances on their cloud platform:

Do you have write access to ggml-org/tmp?

[aldehir]

Do you have write access to ggml-org/tmp?

Does not appear so.

[taronaeo]

Struggling to find a good way to test s390x in my own repo. It seems IBM no longer allows new instances on their cloud platform:

Do you have write access to ggml-org/tmp?

IIRC our IBM Z POWER GHA app didn't whitelist the ggml-org/tmp repository so it would not run there.

I think you can create a branch here instead and it will fire off as per normal :)

[CISC]

Success:
https://github.com/ggml-org/llama.cpp/actions/runs/27870100774/job/82480806825#step:10:877