Add backup image build/publish pipeline with Kopia + Rclone + PostgreSQL 17 client

[Copilot]

This introduces a repository-native container build that produces openproject/backup with current backup tooling (kopia, rclone) and pg_dump from PostgreSQL 17. It also adds automated publishing to Docker Hub from GitHub Actions.

• Image composition

  • Added a multi-stage Dockerfile that:
    • sources kopia from kopia/kopia:latest
    • sources rclone from rclone/rclone:latest
    • uses postgres:17 as the runtime base for PostgreSQL 17 client tools (pg_dump)

• Publish workflow

  • Added .github/workflows/docker-image.yml to build and push on main/master (and manual dispatch).
  • Publishes:
    • openproject/backup:latest
    • openproject/backup:${{ github.sha }}

• Docs alignment

  • Updated README.md to document image contents and the workflow location.

FROM kopia/kopia:latest AS kopia
FROM rclone/rclone:latest AS rclone
FROM postgres:17

COPY --from=kopia /usr/bin/kopia /usr/local/bin/kopia
COPY --from=rclone /usr/local/bin/rclone /usr/local/bin/rclone

Original prompt

This should create a docker image containing the latest versions of kopia, rclone, and postgresql client in version 17 (for pg_dump).

There should be a github workflow building the image and pushing it to openproject/backup:latest on docker hub.