Collapse Set image tag, Build Docker image, and Push image steps into
a single step that computes TAG inline, eliminating the dependency on
$GITHUB_OUTPUT which is not reliably available in Gitea Actions runners.
Also moves registry login before the build+push step for correct ordering.
Closesleeworks-agents/gitea-mobile#25
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>