Fix favicon.ico build bug and modify CI to run on PR (#479)

.github/workflows/dotnetcore.yml

@@ -1,6 +1,6 @@
 name: eShopOnWeb Build and Test
 
-on: [push]
+on: [push, pull_request, workflow_dispatch]
 
 jobs:
   build:
@@ -12,8 +12,10 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.201
+        dotnet-version: 3.1.x
+
     - name: Build with dotnet
       run: dotnet build ./eShopOnWeb.sln --configuration Release
+    
     - name: Test with dotnet
       run: dotnet test ./eShopOnWeb.sln --configuration Release