chore: add updater image configuration to Docker workflow

- Introduced a new job for building and pushing an updater Docker image, dynamically setting its name based on the repository structure.
- Updated existing image name configurations for frontend and backend to strip version numbers, ensuring cleaner image tags.
- Enhanced the Docker workflow with steps for checking out the repository, configuring Buildx, and logging into the Gitea registry.

deploy/updater/validate.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+bash -n "${ROOT}/entrypoint.sh"
+
+jq -e '.targets | type == "array" and length > 0' "${ROOT}/targets.json.example" >/dev/null
+
+for target in backend frontend; do
+  jq -e --arg target "$target" '.targets[] | select(.id == $target) | .image and .container_name and .health.url' \
+    "${ROOT}/targets.json.example" >/dev/null
+done
+
+printf 'updater validation passed\n'