ci: publish images to the Gitea registry, use the gitea context

Release now pushes to git.lucasrguerra.dev.br instead of ghcr.io, tags each
architecture separately (no artifact upload, no gha cache) and joins them with
imagetools. GITHUB_OUTPUT and GITHUB_STEP_SUMMARY stay: they are the runner's
own interface and the GITEA_* equivalents are not set.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -15,7 +15,7 @@ on:
   workflow_call:
 
 concurrency:
-  group: ci-${{ github.ref }}
+  group: ci-${{ gitea.ref }}
   cancel-in-progress: true
 
 jobs: