cfddns: add build/ subdir matching pattern of other custom-image apps

- README.md explains the image is built from the separate fork repo at
  git.alexzaw.dev/alexz/cloudflare-ddns, lists what's in the image, and
  documents how to rebuild + redeploy.
- build.sh clones (or refreshes) /tmp/cfddns-build from the fork via SSH,
  runs docker build, and pushes to git.alexzaw.dev/alexz/cloudflare-ddns:latest.
  Supports IMAGE_NAME/IMAGE_TAG/SOURCE_REPO overrides and passes positional
  args through to docker build (e.g. --no-cache).
- .gitignore for build artifacts.

apps/cfddns/build/README.md

@@ -0,0 +1,66 @@
+# cfddns — Build Files
+
+The Docker image for this app is **not built from this directory**. The
+Dockerfile and Go source live in a separate Gitea-hosted fork at
+[`git.alexzaw.dev/alexz/cloudflare-ddns`](https://git.alexzaw.dev/alexz/cloudflare-ddns)
+(hard fork of [`favonia/cloudflare-ddns`](https://github.com/favonia/cloudflare-ddns)
+with an added `cmd/manager` binary that wraps the original `ddns` updater
+with a web UI for managing the domain list).
+
+The published image is `git.alexzaw.dev/alexz/cloudflare-ddns:latest`.
+
+## What the image contains
+
+- `/bin/ddns` — the unmodified upstream Cloudflare DDNS updater.
+- `/bin/manager` — the wrapper: web UI (HTTP basic auth) on port 8080, YAML
+  config at `/config/cfddns.yaml`, supervises `ddns` as a subprocess and
+  restarts it cleanly on config changes.
+- Base: `alpine:3.23` (needed because the Runtipi healthcheck shells out to
+  `wget --spider` — `scratch` has no userland).
+- Multi-arch: currently `linux/amd64` only. Add `linux/arm64` by extending
+  the buildx build with `--platform`.
+
+## Building
+
+```bash
+cd /etc/runtipi/repos/runtipi/apps/cfddns/build
+./build.sh
+```
+
+This clones (or updates) the fork into `/tmp/cfddns-build`, runs `docker
+build`, and pushes the resulting image to
+`git.alexzaw.dev/alexz/cloudflare-ddns:latest`. Docker push credentials must
+already be present in `/root/.docker/config.json` (the script runs with
+`sudo`).
+
+To target a different tag or registry:
+
+```bash
+IMAGE_TAG=v1-ui2 ./build.sh
+IMAGE_NAME=git.alexzaw.dev/alexz/cfddns IMAGE_TAG=test ./build.sh
+```
+
+To pass extra flags to `docker build` (e.g. `--no-cache`):
+
+```bash
+./build.sh --no-cache
+```
+
+## After building
+
+Follow the standard custom-app deploy chain:
+
+```bash
+cd /etc/runtipi/repos/runtipi
+sudo git push origin main          # only if app definition changed
+cd /etc/runtipi
+sudo ./runtipi-cli appstore update
+sudo ./runtipi-cli app update cfddns:runtipi
+```
+
+The new image is pulled automatically by `app update`.
+
+## Source
+
+- Fork: <https://git.alexzaw.dev/alexz/cloudflare-ddns>
+- Upstream: <https://github.com/favonia/cloudflare-ddns>