Publish Docker images to Docker Hub and GHCR on tag

[HelgeSverre]

Summary

• New root Dockerfile (multi-stage rust:slim-bookworm → debian:bookworm-slim). Includes g++/cmake so the bundled duckdb C++ build succeeds.
• New .dockerignore to keep the build context small.
• New .github/workflows/docker.yml:
  • on tag push v*: build multi-arch (linux/amd64, linux/arm64) and push to both docker.io/<DOCKERHUB_USERNAME>/sql-splitter and ghcr.io/<owner>/sql-splitter
  • on PR touching build inputs: amd64 dry-run, no push (so the next merge isn't blind)
  • workflow_dispatch with a tag input: builds and publishes that ref (use this to backfill old releases)

Tags applied per image (via docker/metadata-action):

• 1.13.5, 1.13, 1, latest (when the ref is the highest semver)
• the literal ref name (v1.13.5) for parity with the GitHub tag

Before merging

Add two repo secrets (Settings → Secrets and variables → Actions):

• DOCKERHUB_USERNAME — your Docker Hub user (e.g. helgesverre)
• DOCKERHUB_TOKEN — a Docker Hub access token (Account Settings → Security → New Access Token, scoped read/write/delete on the repo)

GHCR uses the workflow's GITHUB_TOKEN — no extra secret needed.

After merging

Backfill old releases by running the workflow once per existing tag:

gh workflow run docker.yml --repo HelgeSverre/sql-splitter -f tag=v1.13.4

(I'll do this for all ~30 tags once secrets are set.)

Test plan

• PR check builds the image successfully on amd64
• DOCKERHUB_USERNAME + DOCKERHUB_TOKEN secrets added to repo
• After merge, dispatch a single backfill (e.g. v1.13.5) and verify it appears on https://hub.docker.com/r//sql-splitter
• docker run --rm <user>/sql-splitter:latest --version prints the version
• Backfill remaining tags via workflow_dispatch loop

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