Update dotnetcore.yml

Learning about continuous integration.
- Testing build using multiple versions of .NET Core.

.github/workflows/dotnetcore.yml

@@ -7,11 +7,16 @@ jobs:
 
     runs-on: ubuntu-latest
 
+    strategy:
+      matrix:
+        dotnet: [ '2.1', '3.1' ]
+    name: Dotnet ${{ matrix.dotnet }} build
+
     steps:
-    - uses: actions/checkout@v1
-    - name: Setup .NET Core
-      uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: 2.2.108
-    - name: Build with dotnet
-      run: dotnet build --configuration Release
+      - uses: actions/checkout@v1
+      - name: Setup .NET Core
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: ${{ matrix.dotnet }}
+      - name: Build with dotnet
+        run: dotnet build --configuration Release