Dockerfile: tag base image, add maintainer email

Tagging a base image is good practice regardless. The slim variant also
shaves ~400MB off our final image.

And having a maintainer email is nice.

Author: Will Fleming

Dockerfile

@@ -1,6 +1,5 @@
-FROM node
-
-MAINTAINER Michael R. Bernstein
+FROM node:5.5-slim
+MAINTAINER Michael R. Bernstein <[email protected]>
 
 WORKDIR /usr/src/app/