GitHub Actions: Build a Xamarin.Forms project and create the APK

main.yml

name: Build

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]

jobs:
  # Build App
  build:
    runs-on: windows-latest
    
    env:
      SERVICE_APP_KEY: ${{ secrets.SERVICE_APP_KEY }}

    steps:
      - uses: actions/checkout@v2

      - name: Setup MSBuild
        uses: microsoft/[email protected]        

      - name: Build Solution
        run: msbuild ./MyApplication.sln /restore /p:Configuration=Release

      - name: Create and Sign the APK
        run: msbuild MyApplication\MyApplication.Android\MyApplication.Android.csproj /t:SignAndroidPackage /p:Configuration=Release /p:OutputPath=bin\Release\

      - name: Upload artifact
        uses: actions/upload-artifact@v2
        with:
          name: MyApplication.apk
          path: MyApplication\MyApplication.Android\bin\Release\com.companyname.myapplication-Signed.apk 

That was helpful, here is an update

name: Build android app

on:
  push:
    branches: [ master ]

jobs:
  # Build App
  build:
    runs-on: windows-latest
    
    env:
      SERVICE_APP_KEY: ${{ secrets.SERVICE_APP_KEY }}
      APPLICATION_NAME: MyApplication
      PACKAGE_NAME: com.companyname.myapplication
      
    steps:
      - uses: actions/checkout@v2

      - name: Setup MSBuild
        uses: microsoft/[email protected]

      - name: Build Solution
        run: msbuild ./${{ env.APPLICATION_NAME }}.sln /restore /p:Configuration=Release

      - name: Create and Sign the APK
        run: msbuild ${{ env.APPLICATION_NAME }}.Android\${{ env.APPLICATION_NAME }}.Android.csproj /t:SignAndroidPackage /p:Configuration=Release /p:OutputPath=bin\Release\

      - name: List folder content
        run: dir ${{ env.APPLICATION_NAME }}.Android\bin\Release

      - name: Upload artifact
        uses: actions/upload-artifact@v2
        with:
          name: MyApplication.apk
          path: ${{ env.APPLICATION_NAME }}.Android\bin\Release\${{ PACKAGE_NAME }}-Signed.apk