Publish multi-arch pg_durable Docker images to GHCR on releases and main

[Copilot]

pg_durable currently requires local source builds to try the extension, creating a high setup barrier for evaluation. This PR adds automated publishing of a ready-to-run public image so users can start from docker run instead of compiling Rust/pgrx locally.

• Container publishing workflow

  • Adds .github/workflows/docker-publish.yml to build and push a multi-arch image (linux/amd64, linux/arm64) to ghcr.io/<owner>/pg_durable.
  • Triggers on:
    • push to main
    • version tags (v*)
    • workflow_dispatch
  • Uses Buildx/QEMU and GitHub Actions cache-backed builds.

• Tagging strategy

  • Always publishes traceable tags: pg17, sha-<short_sha>.
  • On main: publishes edge.
  • On release tags (vX.Y.Z): publishes latest, vX.Y.Z, and X.Y.Z.

• README updates

  • Documents the public GHCR image and tag behavior in the Packages section.
  • Adds a direct docker run example for pulling and starting the prebuilt image.

# .github/workflows/docker-publish.yml (tag selection)
if [[ "${GITHUB_REF}" == "refs/heads/main" ]]; then
  tags+=("${IMAGE_NAME}:edge")
fi

if [[ "${GITHUB_REF}" == refs/tags/v* ]]; then
  version="${GITHUB_REF#refs/tags/v}"
  tags+=(
    "${IMAGE_NAME}:${version}"
    "${IMAGE_NAME}:v${version}"
    "${IMAGE_NAME}:latest"
  )
fi

[pinodeca]

I bypassed review rules and merged this one because:

• the refactor of the Docker CI workflow was validated by green run
• I am unable to test the new Docker Publish workflow until it's merged to main.

I will test and open a follow-up PR.