allow setting the port via env var

[averykhoo]

Rather than hardcoding it to port 80 here it would be nice to be able to set the port to something else. For example, if the PORT=80 var is defined by default and the -b arg is dropped from the docker CMD, then gunicorn binds to 0.0.0.0:$PORT by default (docs here), which is more easily configurable.