ci: publish the dev container image to GitHub Packages

[ZhuchkaTriplesix]

Summary

Publishes the dev container as ghcr.io/queryahub/querya-devcontainer (GitHub Packages), tags :latest and :flutter-<version>.

• New workflow devcontainer-image.yml: builds .devcontainer/Dockerfile, smoke-tests it (flutter, docker + compose, clang/cmake/ninja, GTK + libsecret via pkg-config), pushes on main/dev when the Dockerfile changes, and weekly for base-image and apt security updates. Pull requests build and test only, never push.
• devcontainer.json: build.cacheFrom points at the published image, so the first build in a dev container or Codespace reuses the toolchain layers. If the package is unreachable the build falls back to the Dockerfile, so nothing breaks before the first publish.
• docs/dev-containers.md explains the image.
• Image carries org.opencontainers.image.source, which links the package to this repository.

Linked issue

None.

Checklist

• Scope matches the change only (no drive-by refactors)
• PR targets dev
• After the first push to dev: set the package visibility to Public (Packages > querya-devcontainer > Package settings), otherwise anonymous pulls and Codespaces cannot use the cache