chore: Update Docker registry environment variables in CI workflow

### Summary of Changes
- Introduced a new environment variable `REGISTRY_URL` for the Docker registry in the CI workflow for `docker-build.yml`.
- Updated the registry reference to use `REGISTRY_URL` instead of `REGISTRY` for improved clarity and consistency.

### Expected Results
- Enhanced readability and maintainability of the CI workflow by clearly separating the registry URL and its usage.

.gitea/workflows/docker-build.yml

@@ -8,7 +8,8 @@ on:
     branches: [ main, develop ]
 
 env:
-  REGISTRY: https://gittea.kammenstraatha.duckdns.org
+  REGISTRY: gittea.kammenstraatha.duckdns.org
+  REGISTRY_URL: https://gittea.kammenstraatha.duckdns.org
   IMAGE_PREFIX: labfusion
 
 jobs:
@@ -25,7 +26,7 @@ jobs:
     - name: Login to Container Registry
       uses: docker/login-action@v3
       with:
-        registry: ${{ env.REGISTRY }}
+        registry: ${{ env.REGISTRY_URL }}
         username: ${{ secrets.REGISTRY_USERNAME }}
         password: ${{ secrets.REGISTRY_PASSWORD }}