add drone build script

.drone.yml

@@ -0,0 +1,19 @@
+pipeline:
+
+  build:
+    image: alpine:edge
+    commands:
+      - sed -i 's@<VERSION>@${VERSION}@' Dockerfile
+
+  docker:
+    image: plugins/docker
+    repo: bunix42/google-cloud-sdk
+    tags:
+     - ${VERSION}
+    secrets: [ docker_username, docker_password ]
+    when:
+      branch: master
+
+matrix:
+  NC_VERSION:
+    - 193.0.0-alpine