ci: fix Vulkan Docker build failures and GHCR registry timeout

[Ektisad25]

Problem

The Publish Docker image workflow was failing on two jobs:

1. Vulkan job — build exited with code 100 during apt install
2. GHCR token endpoint — net/http: request canceled (Client.Timeout exceeded)

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Changes

.devops/vulkan.Dockerfile

• Changed base image from ubuntu:26.04 → ubuntu:24.04 (stable LTS)
• Split Vulkan SDK deps from glslc install to isolate failures
• Added fallback package installs for glslc since the package name
  differs across Ubuntu releases and GitHub runner images:

apt install -y glslc || apt install -y shaderc || apt install -y shaderc-tools

• Added command -v glslc to verify the binary is available before build

.github/workflows/docker.yml

• Added max-parallel: 2 to the matrix strategy to avoid overloading
  the GHCR token endpoint when multiple Docker images are pushed simultaneously

CMakePresets.json

• Downgraded "version" from 4 → 3 for broader CMake compatibility
• Added new configure preset x64-linux-gcc-lowmem-release with server,
  tests, and examples disabled — suitable for memory-constrained build environments
• Added matching build preset x64-linux-gcc-lowmem-release-cli that
  builds only llama-cli with "jobs": 1 to minimize memory usage

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Commits

Hash	Message
036a2d1	chore: update CMakePresets for low-memory build preset
8f62a92	ci: stabilize Vulkan Docker image build on GitHub Actions
789d827	ci: add glslc package fallbacks for Vulkan Docker build
37a8fec	ci: reduce parallel docker pushes to mitigate GHCR timeouts

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Testing

Validated locally. The Vulkan Dockerfile now handles package naming
differences across Ubuntu 22.04 and 24.04 runner images. The max-parallel
cap prevents GHCR token endpoint exhaustion under concurrent pushes.