ci: add fastattn backend to Docker build matrix

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: raylim

.github/workflows/docker.yml

@@ -25,6 +25,7 @@ jobs:
           - torch-cpu
           - tensorflow-gpu
           - tensorflow-cpu
+          - fastattn
 
     steps:
     - name: Checkout repository