Avoid broken Radicale releases in CI

Author: pat

.github/workflows/main.yml

@@ -18,8 +18,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    # Locking to 3.1.9 as 3.1.9-1 and 3.1.9-2 are both broken (and nothing newer
+    # has been released at this point).
     - name: Run Radicale
-      run: docker run -v ${{ github.workspace }}/spec/data/radicale:/var/radicale --name radicale --publish 8000:8000 --detach xlrl/radicale
+      run: docker run -v ${{ github.workspace }}/spec/data/radicale:/var/radicale --name radicale --publish 8000:8000 --detach xlrl/radicale:3.1.9
 
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1