feat(slugrunner): update stack to heroku-18 (#7)

duanhongyi · Cryptophobia · commit b67255022bd1 · 2018-10-30T13:48:40.000-04:00
diff --git a/rootfs/Dockerfile b/rootfs/Dockerfile
@@ -1,4 +1,4 @@
-FROM heroku/cedar:14
+FROM heroku/heroku:18
 
 RUN mkdir /app
 RUN addgroup --quiet --gid 2000 slug && \
