Switched to lock at node 7.8

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:7.9
+FROM node:7.8
 
 # Create app directory
 RUN mkdir -p /usr/src/app

INSTALL.md

@@ -174,8 +174,8 @@ and testing purposes.
 
 There are some runtime requirements for running Talk from source:
 
-- [Node](https://nodejs.org/) v7.9 or later
-- [Yarn](https://yarnpkg.com/) v0.22.0 or later
+- [Node](https://nodejs.org/) ~7.8
+- [Yarn](https://yarnpkg.com/) ^0.22.0
 
 _Please be sure to check the versions of these requirements. Incorrect versions
 of these may lead to unexpected errors!_

package.json

@@ -182,6 +182,6 @@
     "webpack": "^2.3.1"
   },
   "engines": {
-    "node": "^7.9.0"
+    "node": "^7.8.0"
   }
 }