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>

docs/DEPLOYMENT.md

@@ -176,7 +176,7 @@ have propagated and the IP has some sending history.
 
 ## Upgrading
 
-Images are published to GHCR for `linux/amd64` and `linux/arm64` on every merge
+Images are published to the Gitea container registry for `linux/amd64` and `linux/arm64` on every merge
 to `main`.
 
 ```bash
@@ -198,7 +198,7 @@ every release, which is what forces the pull.
     IMAGE_TAG=sha-<full commit sha>
 
 The tags are published by `release.yml`; `docker buildx imagetools inspect
-ghcr.io/<owner>/lambdamail/webmail:latest` shows what `latest` currently points
+git.lucasrguerra.dev.br/<owner>/lambdamail/webmail:latest` shows what `latest` currently points
 at.
 
 **Pinning has a second edge: bump it on every release.** A pinned tag is an