add drone CI

.drone.yml

@@ -0,0 +1,17 @@
+---
+kind: pipeline
+name: default
+
+steps:
+  - name: build and publish image
+    image: plugins/docker
+    settings:
+      repo: eumau/openldap
+      auto_tag: true
+      username:
+        from_secret: dockerhub_username
+      password:
+        from_secret: dockerhub_password
+    when:
+      branch:
+        - master