Allow building Docker image via `docker-compose build`

docker-compose.yml

@@ -31,6 +31,7 @@ services:
     ### configuration options for this image.
 
   sidekiq:
+    build: .
     image: asciinema/asciinema.org
     container_name: asciinema_worker
     links:
@@ -51,6 +52,7 @@ services:
     ### options for this image.
 
   web:
+    build: .
     image: asciinema/asciinema.org
     container_name: asciinema_web
     links:
@@ -66,6 +68,7 @@ services:
       - /tmp/asciinema/uploads:/app/uploads
 
   db_init:
+    build: .
     image: asciinema/asciinema.org
     links:
       - redis