Create test-build.yml

Add manual dispatch for testing build in Docker image

.github/workflows/test-build.yml

@@ -0,0 +1,16 @@
+name: Run test build with esp-idf-rust image from DockerHub
+
+on: workflow_dispatch
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repo
+      uses: actions/checkout@v2
+      with:
+        submodules: 'recursive'
+    - name: esp-idf build
+      uses: espressif/esp-idf-ci-action@rust-latest