Replace the old Docker-in-Docker build approach (plugins/docker) with
the modern buildx plugin that natively supports BuildKit. This fixes:
- "Cannot connect to the Docker daemon" errors (no DinD needed)
- "the --mount option requires BuildKit" errors (buildx = BuildKit)
- OOM in publish step (skopeo no longer needed, buildx pushes directly)
Also removes the intermediate build-tag/skopeo-copy/publish dance —
buildx pushes both versioned and latest tags in a single step.
Replace .drone.yml with .woodpecker/ pipeline configs (frontend.yml, api.yml).
Convert Drone env vars to Woodpecker equivalents (CI_PIPELINE_NUMBER, CI_COMMIT_SHA),
use woodpeckerci/plugin-git for clone with retry, woodpeckerci/plugin-slack for
notifications, and plugins/docker for image builds. Update all docs and skills.
