Switch to the official Node.js Docker image

The official image already contains Yarn and everything we need so there's no need for a custom image.

docker-compose.yml

@@ -3,6 +3,8 @@ version: '2'
 services:
 
   app:
-    build: docker/app
+    image: node:8
     volumes:
       - ./:/app
+    working_dir: /app
+    command: 'yarn cron'

docker/app/Dockerfile

@@ -1,13 +0,0 @@
-FROM ubuntu:16.04
-
-# NodeJS / Yarn
-RUN apt-get update \
-    && apt-get install -y curl \
-    && curl -sL https://deb.nodesource.com/setup_6.x | bash - \
-    && curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
-    && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list \
-    && apt-get update \
-    && apt-get install -y nodejs yarn bzip2 libfontconfig
-
-WORKDIR /app
-CMD ["yarn", "cron"]