add docker build step

.github/workflows/docker.yml

@@ -0,0 +1,20 @@
+name: Docker
+
+on:
+  push:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Login to DockerHub
+      uses: docker/login-action@v2
+      with:
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
+    - name: Build and push
+      uses: docker/build-push-action@v3
+      with:
+        push: true
+        tags: kerberos/agent:beta