Add release build workflow

.github/workflows/release.yml

@@ -0,0 +1,53 @@
+on:
+  push:
+    tags:
+    - "v[0-9]+.[0-9]+.[0-9]+"
+env:
+  DOTNET_NOLOGO: true
+  DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
+  DOTNET_CLI_TELEMETRY_OPTOUT: true
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Verify commit exists in origin/master
+      run: |
+        git fetch --no-tags --prune --depth=1 origin '+refs/heads/*:refs/remotes/origin/*'
+        git branch --remote --contains | grep origin/master
+    - name: Set VERSION variable from tag
+      run: echo "VERSION=${GITHUB_REF/refs\/tags\/v/}" >> $GITHUB_ENV
+    - name: Pack
+      run: |
+        dotnet pack FB2Library/FB2Library.csproj \
+          -p:NuspecFile=../nuget/FB2Library.nuspec \
+          -p:NuspecProperties="version=${VERSION}" \
+          --configuration Release \
+          --verbosity normal \
+          --include-symbols \
+          --include-source \
+          --output .
+    - name: Upload Artifact
+      uses: actions/upload-artifact@v2
+      with:
+        name: nupkg
+        path: '*.nupkg'
+
+  deploy-nuget:
+    needs: build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download Artifact
+        uses: actions/download-artifact@v1
+        with:
+          name: nupkg
+      - name: Push to NuGet
+        run: |
+          dotnet nuget push ./nupkg/FB2Library.*.nupkg \
+            --source ${FEED_URL} \
+            --api-key ${FEED_KEY}
+        env:
+          FEED_URL: https://api.nuget.org/v3/index.json
+          FEED_KEY: ${{ secrets.NUGET_KEY }}