introduce github actions

.github/workflows/main.yml

@@ -0,0 +1,16 @@
+name: Build
+
+on: [pull_request, push]
+
+jobs: 
+  build:
+    container:
+      image: ghcr.io/armmbed/mbed-os-env:latest
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Compile Starter Project
+        run: make
+        working-directory: 'apps/beats-time/make'
+