Add NuGet publishing job for tags

.github/workflows/build.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
      - main
+    tags:
+    - v*
   pull_request:
     branches:
       - main
@@ -31,3 +33,20 @@ jobs:
         with:
           name: GovUk.OneLogin.AspNetCore.nupkg
           path: src/GovUk.OneLogin.AspNetCore/bin/Release/*.nupkg
+
+  release:
+    name: "Release"
+    runs-on: ubuntu-latest
+    needs: build
+    if: startsWith(github.ref, 'refs/tags/v')
+
+    steps:
+    - name: Download package artifact
+      uses: actions/download-artifact@v3
+      with:
+        name: GovUk.OneLogin.AspNetCore.nupkg
+
+    - name: Publish package to NuGet
+      run: dotnet nuget push **/*.nupkg --api-key $NUGET_TOKEN --source https://api.nuget.org/v3/index.json --skip-duplicate
+      env:
+          NUGET_TOKEN: ${{ secrets.NUGET_TOKEN }}