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/build.sh

@@ -0,0 +1,39 @@
+#!/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"