fix: bake version into Docker image at build time

- Add VERSION build arg to Dockerfile
- Pass --build-arg VERSION in release workflow
- Remove VERSION env override from docker-compose files
- Version is now immutable inside the image, no runtime env var needed

.gitea/workflows/release.yml

@@ -16,7 +16,7 @@ jobs:
         run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login git.cqre.net -u ${{ github.actor }} --password-stdin 2>&1 | grep -v "WARNING! Your credentials are stored unencrypted"
 
       - name: Build Docker image
-        run: docker build ./backend --tag git.cqre.net/cqrenet/aoc-backend:${{ gitea.ref_name }}
+        run: docker build ./backend --build-arg VERSION=${{ gitea.ref_name }} --tag git.cqre.net/cqrenet/aoc-backend:${{ gitea.ref_name }}
 
       - name: Push Docker image
         run: docker push git.cqre.net/cqrenet/aoc-backend:${{ gitea.ref_name }}