Add check of used yarn version (#81)

* Add yarn version check to action

* Add workflow_dispatch what allows to trigger workflow manually

.github/workflows/master.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - master
+  workflow_dispatch:
 
 env:
   AZURE_WEBAPP_NAME: pos-backend-staging # set this to your application's name
@@ -16,6 +17,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@master
+      - name: Print Yarn version
+        run: yarn --version
       - name: Use Node.js ${{ env.NODE_VERSION }}
         uses: actions/setup-node@v1
         with: