Fixing Docker invalid reference format errors in GitHub Actions

Docker invalid reference format errors in GitHub Actions occur when the client rejects an image name locally, before any connection to a registry, because it violates the required reference grammar. The cause is usually visible in the printed reference, such as an uppercase letter in the repository name, a variable that expanded to nothing, or a doubled separator in the tag. Because the check happens in the client, these failures appear within a second and do not clear on retry. The article recommends computing a lowercase image name once, publishing it as a step output, and reusing it across jobs, for example with docker/metadata-action.