Add caching for build target directory

.drone.yml

@@ -3,6 +3,16 @@ type: docker
 name: default
 
 steps:
+- name: restore-target-cache
+  image: drillster/drone-volume-cache
+  volumes:
+  - name: target-cache
+    path: /cache
+  settings:
+    restore: true
+    mount:
+      - ./target
+
 - name: test
   image: rust:1
   volumes:
@@ -12,7 +22,20 @@ steps:
   - cargo build --verbose --all
   - cargo test --verbose --all
 
+- name: rebuild-target-cache
+  image: drillster/drone-volume-cache
+  volumes:
+  - name: target-cache
+    path: /cache
+  settings:
+    rebuild: true
+    mount:
+      - ./target
+
 volumes:
 - name: cargo-registry
   host:
     path: /var/lib/drone/sprinklers_rs/cargo-registry
+- name: target-cache
+  host:
+    path: /var/lib/drone/sprinklers_rs/target-cache