Pin to 2.7-slim-jessie

2.7-slim was repointed to 2.7-slim-stretch, which is missing
a number of dependencies we need to run the app.

See docker-library/python#293

Author: sloria

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:2.7-slim
+FROM python:2.7-slim-jessie
 
 ENV GOSU_VERSION=1.10 \
     NODE_ENV=production \