Updated docker build script

[djthorpe]

This pull request refactors the GitHub Actions workflow for building and publishing Docker images. The main improvements focus on simplifying variable management, enhancing consistency with environment variables, and streamlining the build and manifest creation process.

Workflow and Variable Management:

• Removed the separate var job and replaced custom variable passing with a centralized DOCKER_REPO environment variable, simplifying workflow logic and improving maintainability.
• Updated job dependencies to use outputs directly from the build job, eliminating unnecessary complexity in job chaining.

Build and Manifest Process:

• Changed the build matrix to explicitly specify both the release tag and "latest" for Docker image tagging, ensuring both tags are created reliably.
• Updated build steps to use BUILDKIT_PROGRESS=plain for improved logging and to output the Docker image tag for downstream jobs.
• Refactored manifest creation and annotation steps to consistently use the DOCKER_REPO environment variable, aligning image references and making the workflow more robust.

[Copilot]

Pull request overview

This PR refactors the GitHub Actions publish workflow that builds and publishes multi-arch Docker images to GHCR, consolidating image naming/tagging under a single DOCKER_REPO env var and simplifying the build→manifest flow.

Changes:

• Removes the separate “var” job and centralizes the image repository as env.DOCKER_REPO.
• Exposes the computed image version tag from the build job for downstream consumption.
• Streamlines manifest creation to publish both the version tag and latest consistently using the same repo/tag scheme.