Specify the port as to play with Heroku.

This probably breaks things running locally. Let's talk about how we can use the 8888 for local
development. Please refer to https://devcenter.heroku.com/articles/container-registry-and-runtime
for information.

NOTE: the EXPOSE keyword is not respected on heroku because they control the ports. Hence the
environment variable PORT.

Signed-off-by: Kevin Fitzpatrick <kevin@influxdb.com>

Dockerfile

@@ -8,4 +8,6 @@ ADD mrfusion /mrfusion
 
 EXPOSE 8888
 
-CMD ["/mrfusion", "--port=8888"]
+# ENV PORT ||= 8888
+
+CMD ["/mrfusion", "--port=$PORT"]