Refactor CI/CD workflows to standardize Docker image naming and improve build process

[tombiddulph]

This pull request updates the GitHub Actions workflows for CI and release to improve Docker image build and tagging processes, and to ensure image names are always lowercase (as required by the container registry). It also adds a new job to verify Docker image builds on pull requests without pushing them.

Workflow improvements and Docker build enhancements:

• Added a docker-build job to .github/workflows/ci.yml that builds the Docker image on pull requests to verify it compiles, but does not push the image.
• Updated both the docker-push job in .github/workflows/ci.yml and the Docker build job in .github/workflows/release.yml to set and use a lowercase IMAGE_NAME environment variable, ensuring compatibility with registry requirements. [1] [2]
• Updated Docker image tagging in both workflows to use the lowercased image name output from the new step, instead of referencing github.repository directly. [1] [2]
• Updated the release notes in .github/workflows/release.yml to reference the new lowercased image name in the Docker pull instructions.

Minor workflow cleanups:

• Removed a redundant comment and whitespace in the build-and-test job in .github/workflows/ci.yml.
• Removed an unnecessary comment in the Checkout step regarding fetch-depth in .github/workflows/ci.yml.