Attempt to build documentation image in CI.

.drone.yml

@@ -1,5 +1,22 @@
 ---
 kind: pipeline
+name: documentation
+
+steps:
+- name: build
+  image: plugins/docker
+  settings:
+    username:
+      from_secret: docker_username
+    password:
+      from_secret: docker_password
+    repo: witten/borgmatic-docs
+    dockerfile: docs/Dockerfile
+  when:
+    branch:
+      - master
+---
+kind: pipeline
 name: python-3-5-alpine-3-9
 
 steps: