Add docker build test workflow

.github/workflows/docker-build.yml

@@ -0,0 +1,17 @@
+name: Docker Build Test
+
+on:
+  push:
+    branches: [ mainline ]
+  pull_request:
+    branches: [ mainline ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build the Docker image
+      run: docker build . -t "ktf:build" -f tools/docker/Dockerfile --build-arg USER_ID=$(id -u) --build-arg GROUP_ID=$(id -g) --build-arg USER=$USER
+    - name: Run the Docker image
+      run: docker run -e UNITTEST=1 -v $(pwd):$(pwd) -w $(pwd) "ktf:build" bash -c "make -j clean boot.iso"