Add .gitea/workflows/docker.yml that builds the image via Dockerfile.dev
and pushes it to the Gitea container registry at
git.hackner.dev/thomas/rustdesk-api on pushes to master and on v*.*.* tags.
Remove the GitHub Actions workflows (build.yml, build_test.yml): they push
to Docker Hub and GHCR (not wanted) and rely on GitHub-only features
(GITHUB_TOKEN, GitHub Releases, changelogithub) that do not work on Gitea.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
