Migrate Android CI to shared org-level Tauri CI base image

[ScottMorris]

Context

The build-android job in release-build.yml currently uses a per-repo CI base image:

container:
  image: ghcr.io/${{ github.repository }}/ci-base:latest

A shared org-level image is being created at ghcr.io/liminal-hq/tauri-ci-base (tracked in liminal-hq/tauri-plugins-workspace).

Proposal

1. Update release-build.yml build-android job to use ghcr.io/liminal-hq/tauri-ci-base:latest
2. Update .devcontainer/Dockerfile to use the shared image as base
3. Retire build-ci-image.yml (or repurpose if repo-specific layers are needed)

Acceptance Criteria

• release-build.yml references the shared image
• .devcontainer uses shared image as base
• Release build passes in draft mode with the shared image
• build-ci-image.yml removed or repurposed