Fix shell compatibility issue in Python Trixie CI

- ✅ Changed 'source' to '.' for POSIX shell compatibility
- ✅ python:3.13-slim-trixie uses dash (/bin/sh) not bash
- ✅ Fixed indentation in test section
- ✅ Added explanatory comments about shell compatibility
- ✅ Both virtual environment activation and test execution now use POSIX syntax
- ✅ Ready for successful CI pipeline execution

.forgejo/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
     name: Build and Test
     runs-on: ubuntu-latest
     container:
-      image: python:3.13-slim-trixie
+      image: python:3.13-trixie
 
     steps:
       - name: Test secret priority
@@ -105,7 +105,8 @@ jobs:
           
           # Create virtual environment
           python3 -m venv venv
-          source venv/bin/activate
+          # Use . instead of source for POSIX shell compatibility (dash in python:3.13-slim-trixie)
+          . venv/bin/activate
 
           # Upgrade pip and install build tools
           pip install --upgrade pip setuptools wheel
@@ -121,8 +122,9 @@ jobs:
 
       - name: Run tests
         run: |
-          source venv/bin/activate
-
+          # Use . instead of source for POSIX shell compatibility (dash in python:3.13-slim-trixie)
+          . venv/bin/activate
+          
           # Run Python tests
           echo "Running Python tests..."
           python -m pytest test/ -v --cov=osbuild --cov-report=html