feat: add debian:13-slim runner image for Gitea Actions

Self-contained Dockerfile under linux/debian/ that bakes runner user,
Docker CE, Node.js v20+v24, latest Go, yq, and act env vars. Uses
Debian's git-lfs and Docker CE from download.docker.com. Published to
git.ipng.ch/ipng/gitea-debian-runner:13-slim. README documents build
and push commands (single-arch and multi-arch via buildx).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -66,6 +66,47 @@
   - `ghcr.io/catthehacker/ubuntu:custom-24.04`
   - `ghcr.io/catthehacker/ubuntu:custom-latest`
 
+## IPng Debian Runner
+
+A `debian:13-slim`-based variant of the `act`/`runner` image, intended for use with Gitea Actions. Compared to the Ubuntu images it adds `rsync`, ships `git-lfs` from Debian's archive (rather than the packagecloud repo), and uses Docker CE from `download.docker.com` instead of Microsoft's moby builds.
+
+Source: [`linux/debian/Dockerfile`](./linux/debian/Dockerfile). It is a single self-contained Dockerfile (no helper scripts) and bakes the `runner` user, Docker CE, Node.js LTS (v20 + v24), the latest stable Go release from go.dev, `yq`, and the `act`/runner environment variables (`ImageOS=debian13`, tool cache paths, etc.).
+
+Published to `git.ipng.ch/ipng/gitea-debian-runner:13-slim`.
+
+### Build
+
+```sh
+docker build --load \
+  --file linux/debian/Dockerfile \
+  --tag git.ipng.ch/ipng/gitea-debian-runner:13-slim \
+  --build-arg BUILD_DATE="$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+  --build-arg BUILD_REF="$(git rev-parse HEAD)" \
+  .
+```
+
+`--load` makes the image available to the local docker daemon — required when buildx uses the `docker-container` driver (the default on multi-platform setups).
+
+For multi-arch (amd64 + arm64) using buildx, building and pushing in one step:
+
+```sh
+docker buildx build \
+  --file linux/debian/Dockerfile \
+  --platform linux/amd64,linux/arm64 \
+  --tag git.ipng.ch/ipng/gitea-debian-runner:13-slim \
+  --build-arg BUILD_DATE="$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+  --build-arg BUILD_REF="$(git rev-parse HEAD)" \
+  --push \
+  .
+```
+
+### Push (single-arch)
+
+```sh
+docker login git.ipng.ch
+docker push git.ipng.ch/ipng/gitea-debian-runner:13-slim
+```
+
 ## [`ubuntu-20.04` has been deprecated and images for that environment will not be updated anymore](https://github.com/actions/runner-images/pull/11748)
 
 ## [`ubuntu-18.04` has been deprecated and images for that environment will not be updated anymore](https://github.com/actions/runner-images/issues/6002)