docker/unified: vulkan build fixes

[mostlygeek]

multiple fixes to vulkan build:

• use ubuntu 26.04 to be compatible with AMD 395+ (Strix halo) hardware
• add home directory in container
• fix stable-diffusion install to actually enable vulkan

[coderabbitai]

Walkthrough

This PR adds conditional backend build controls via workflow_dispatch inputs to the GitHub Actions workflow, upgrades the Vulkan builder's Ubuntu base image from 22.04 to 26.04, replaces LunarG Vulkan SDK installation with native Ubuntu packages, adds --break-system-packages to pip dependencies, adjusts user creation flags, and enables the SD_VULKAN CMake option during Vulkan builds.

Changes

Cohort / File(s)	Summary
Workflow Configuration .github/workflows/unified-docker.yml	Added build_cuda and build_vulkan boolean workflow_dispatch inputs (both default true), replacing fixed backend matrix with conditional exclusion logic to selectively skip backends based on input values.
Docker Image Setup docker/unified/Dockerfile	Upgraded Vulkan base image to ubuntu:26.04, replaced LunarG Vulkan SDK installation with Ubuntu native packages (libvulkan-dev, glslang-tools, spirv-tools, vulkan-validationlayers, glslc). Added --break-system-packages to pip install and changed llama-swap user creation from --no-create-home to --create-home.
Build Script docker/unified/install-sd.sh	Added -DSD_VULKAN=ON CMake flag when BACKEND is set to vulkan, alongside existing GGML_VULKAN=ON configuration.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• docker/unified: add cuda based unified container #597: Modifies the same files (workflow, Dockerfile, install-sd.sh) with overlapping backend and build flag logic.
• .github,docker/unified: improve caching and fix bugs #598: Updates the same workflow and Dockerfile with related changes to user handling and workflow input behavior.
• .github,docker/unified: include vulkan build #599: Implements backend-splitting changes in unified Docker CI/workflow and install scripts that this PR builds upon with conditional input controls.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'docker/unified: vulkan build fixes' accurately describes the main changes across the PR, which focus on fixing Vulkan builds through container updates, build flags, and workflow controls.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The pull request description accurately describes the key changes: Ubuntu 26.04 upgrade for AMD hardware compatibility, home directory fix for shader caching, and Vulkan build flag enabling in stable-diffusion install script.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch unified-vulkan-update-base

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