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/Dockerfile

@@ -0,0 +1,23 @@
+# syntax=docker/dockerfile:1
+
+FROM docker:27-cli
+
+RUN apk add --no-cache bash curl jq coreutils
+
+WORKDIR /updater
+
+COPY entrypoint.sh /updater/entrypoint.sh
+COPY targets.json.example /etc/updater/targets.json
+
+RUN chmod +x /updater/entrypoint.sh
+
+ENV TARGETS_FILE=/etc/updater/targets.json
+ENV STATE_FILE=/state/updater-state.json
+ENV POLL_INTERVAL_SECONDS=300
+ENV HEALTH_TIMEOUT_SECONDS=120
+ENV STOP_TIMEOUT_SECONDS=30
+ENV REGISTRY=git.shts.su
+
+VOLUME ["/state"]
+
+ENTRYPOINT ["/updater/entrypoint.sh"]