Add CKAN 2.12 image

[vojneski]

CKAN upgraded to 2.12.0

Added a new Docker image for CKAN 2.12.0 and switched the default compose stack to it, following the existing per-version pattern (images/ckan/2.10, images/ckan/2.11).

Changes

• Added images/ckan/2.12/ (Dockerfile + setup/ scripts), copied from 2.11 with IMAGE_TAG bumped to 2.12.0 (pulls the upstream ckan-2.12.0 git tag)
• compose/config/.global-env: CKAN_VERSION set to 2.12.0
• Updated version examples in README.md and CLAUDE.md
• No CI/workflow changes needed — the build/lint/publish matrix and scripts/update_dockerfiles.sh discover version directories dynamically

Verification

• docker build --tag ghcr.io/keitaroinc/ckan:2.12.0 images/ckan/2.12 — builds successfully
• Container boots; pip show ckan inside the image confirms Version: 2.12.0
• docker compose config resolves all CKAN services to ghcr.io/keitaroinc/ckan:2.12.0

Release

• Commit: ebf8eb1 Add CKAN 2.12 image
• Tag: `2.12.0

[mihajlo-kuzmanoski]

LGTM

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