Parallelise Dockerfile stages and enable CI layer caching

Split runtime apt-get into a separate stage so BuildKit runs it
concurrently with the builder's apt-get + pip install. Add cache_from
to the Drone CI API build step to reuse layers from the previous image.

.drone.yml

@@ -58,6 +58,7 @@ steps:
       dockerfile: crawler/Dockerfile
       context: crawler/
       auto_tag: true
+      cache_from: viktorbarzin/realestatecrawler:latest
 
   - name: Update deployment
     image: alpine