Cache dependencies on CI

https://florian-latifi.at/2018/02/21/using-drone-to-build-and-deploy-a-jekyll-site/
https://git.florian-latifi.at/flortsch/jekyll-blog/src/branch/master/.drone.yml

.drone.yml

@@ -3,12 +3,35 @@ type: docker
 name: CI build
 
 steps:
+  - name: restore-cache
+    image: drillster/drone-volume-cache
+    volumes:
+    - name: cache
+      path: /cache
+    settings:
+      restore: true
+      mount:
+      - vendor
   - name: rspec
     image: guildeducation/rails:2.7.1-12.19.0
     commands:
-      - bundle install --jobs=3 --retry=3
+      - bundle install --jobs=3 --retry=3 --deployment
       - yarn install
       - bundle exec rspec
     when:
       branch:
         - master
+  - name: rebuild-cache
+    image: drillster/drone-volume-cache
+    volumes:
+    - name: cache
+      path: /cache
+    settings:
+      rebuild: true
+      mount:
+      - vendor
+
+volumes:
+  - name: cache
+    host:
+      path: /var/lib/drone/tmp