added drone config

.drone.yml

@@ -0,0 +1,20 @@
+kind: pipeline
+name: build
+
+trigger:
+  branch:
+    - master
+  event:
+    - push
+
+steps:
+  - name: docker
+    image: plugins/docker
+    pull: true
+    settings:
+      username:
+        from_secret: docker_username
+      password:
+        from_secret: docker_password
+      repo: vikunja/flutter-build
+      auto_tag: true