ci: Speed up build with cache

.drone.yml

@@ -17,13 +17,13 @@ steps:
     commands:
       # Debug
       - pwd
-      - mkdir -p _artifacts
+      - mkdir -p _artifacts _cache
 
   - name: "build:executable"
     image: crystallang/crystal:1.5.0
     volumes:
-      - name: cache
-        path: /stupid
+      - name: crystal_cache
+        path: /drone/src/_cache
     commands:
       # Debug
       - pwd
@@ -37,15 +37,21 @@ steps:
     volumes:
       - name: dockersock
         path: /var/run
-      - name: cache
-        path: /stupid
+      - name: docker_cache
+        path: /drone/src/_cache
     commands:
+      # Prepare
       - apk add make
+      - test -f _cache/docker-image.tar && docker load < _cache/docker-image.tar
+      # Build
       - make docker-build
       - make docker-save > _artifacts/docker-image.tar
+      - make docker-save > _cache/docker-image.tar
 
 volumes:
-  - name: cache
+  - name: crystal_cache
+    temp: {}
+  - name: docker_cache
     temp: {}
   - name: dockersock
     temp: {}