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/.gitignore

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+# Anything Docker / build temp products created in this dir during a build.
+*.tar
+*.tar.gz

apps/cfddns/build/README.md

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+# 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>

apps/cfddns/build/build.sh

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+#!/bin/bash
+# Build and push the cfddns Docker image.
+# The actual Dockerfile + Go source lives in a separate Gitea repo:
+#   https://git.alexzaw.dev/alexz/cloudflare-ddns
+# This script clones (or updates) that repo into /tmp/cfddns-build and runs
+# `docker build` + `docker push`. Any positional args are forwarded to
+# `docker build` (e.g. `./build.sh --no-cache`).
+
+set -euo pipefail
+
+IMAGE_NAME="${IMAGE_NAME:-git.alexzaw.dev/alexz/cloudflare-ddns}"
+IMAGE_TAG="${IMAGE_TAG:-latest}"
+SOURCE_REPO="${SOURCE_REPO:-ssh://git@git.alexzaw.dev:222/alexz/cloudflare-ddns.git}"
+WORKDIR="${WORKDIR:-/tmp/cfddns-build}"
+
+echo "Building ${IMAGE_NAME}:${IMAGE_TAG} from ${SOURCE_REPO}..."
+
+if [ -d "${WORKDIR}/.git" ]; then
+  echo "Refreshing existing clone at ${WORKDIR}..."
+  git -C "${WORKDIR}" fetch --all --tags
+  git -C "${WORKDIR}" reset --hard origin/main
+else
+  echo "Cloning ${SOURCE_REPO} -> ${WORKDIR}..."
+  rm -rf "${WORKDIR}"
+  git clone "${SOURCE_REPO}" "${WORKDIR}"
+fi
+
+cd "${WORKDIR}"
+
+sudo docker build "$@" -t "${IMAGE_NAME}:${IMAGE_TAG}" .
+sudo docker push "${IMAGE_NAME}:${IMAGE_TAG}"
+
+echo ""
+echo "Built and pushed ${IMAGE_NAME}:${IMAGE_TAG}"
+echo ""
+echo "Next: deploy via runtipi-cli"
+echo "  cd /etc/runtipi"
+echo "  sudo ./runtipi-cli appstore update"
+echo "  sudo ./runtipi-cli app update cfddns:runtipi"