Add drone caching

.drone.yml

@@ -3,10 +3,48 @@ type: kubernetes
 name: default
 
 steps:
+  - name: restore
+    image: plugins/s3-cache
+    settings:
+      pull: true
+      endpoint: https://s3.voidcorp.nl
+      access_key:
+        from_secret: s3accesskey
+      secret_key:
+        from_secret: s3secretkey
+      restore: true
+
   - name: test
     image: rust:1.56
     environment:
-      CARGO_HOME: ./.cargo_home
+      CARGO_HOME: .cargo_home
     commands:
       - cargo build --verbose --all
       - cargo test --verbose --all
+
+  - name: rebuild
+    image: plugins/s3-cache
+    settings:
+      pull: true
+      endpoint: https://s3.voidcorp.nl
+      access_key:
+        from_secret: s3accesskey
+      secret_key:
+        from_secret: s3secretkey
+      rebuild: true
+      mount:
+        - .cargo_home
+      when:
+        event: push
+
+  - name: flush
+    image: plugins/s3-cache
+    settings:
+      pull: true
+      endpoint: https://s3.voidcorp.nl
+      access_key:
+        from_secret: s3accesskey
+      secret_key:
+        from_secret: s3secretkey
+      flush: true
+      flush_age: 14