ci: build Docker image on every push, only push to GHCR on release

[Copilot]

All post-build steps were gated by IS_RELEASE == 'true', causing the Docker image build to be skipped on every non-main/master push — including the Docker Buildx setup and the OWNER env var needed for image tags.

Changes

• Set release env — removed IS_RELEASE guard; OWNER must always be set for Docker tag references
• Set up Docker Buildx — removed IS_RELEASE guard; runs unconditionally
• Build and push image:latest — removed if guard; build always runs, push is conditional:

- name: Build and push image:latest
  uses: docker/build-push-action@v6
  with:
      push: ${{ env.IS_RELEASE == 'true' }}  # build always, push only on release
      platforms: linux/amd64,linux/arm64
      tags: ghcr.io/${{ env.OWNER }}/${{ env.IMAGE_NAME }}:latest

• Login to GitHub Container Registry — remains release-only (no push needed on feature branches)
• Build and push image:tags — unchanged, still requires a new tag from create-tag-action

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[Copilot]

Pull request overview

This PR updates the GitHub Actions CI workflow to consistently use a lowercase GitHub repository owner when referencing GHCR image paths, preventing issues with Docker/GHCR name normalization.

Changes:

• Add a step that computes a lowercase owner value from github.repository_owner and exposes it via step outputs.
• Update the release notes’ Docker command examples to use the lowercase owner for ghcr.io/... image references.
• Update the Docker image tagging logic to publish images under ghcr.io/<lowercase-owner>/<image>:....

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