Enhance Docker build workflow by adding virtual environment setup

- Introduced a Python virtual environment for dependency management.
- Updated pip installation commands to use the virtual environment context.

.gitea/workflows/docker-build.yml

@@ -27,8 +27,10 @@ jobs:
 
       - name: Python-Abhängigkeiten prüfen (pip-audit)
         run: |
+          python -m venv .venv
+          . .venv/bin/activate
           python -m pip install --upgrade pip
-          pip install pip-audit
+          python -m pip install pip-audit
           pip-audit -r requirements.txt
 
       - name: Docker Buildx einrichten