Adds .gitea/workflows/testing.yml, which builds and pushes only the
gitea.it-drui.de/viewit/kx-bridge:testing (+ :testing-<shortsha>) image on a
push to the `testing` branch.
Strictly isolated from nightly/master by construction:
- triggers only on push to `testing` (plus workflow_dispatch), no cron
- contains no git push / tag / release step, so it never writes back to any
branch (unlike nightly.yml, which resets NIGHTLY_CHANGELOG.md back to
nightly)
- uses its own :testing* image tags, never overwriting :nightly / :latest
Conversely, the existing workflows don't react to testing pushes:
nightly.yml is bound to the nightly branch, release.yml to v* tags,
pr-check.yml to PRs against nightly. So a testing push can't trigger any of
them either.