Add nginx log file

[lpsinger]

This makes it easier to diagnose nginx configuration errors. See skyportal/skyportal#2292.

[acrellin]

@lpsinger we already have nginx logs, e.g. https://github.com/cesium-ml/baselayer/blob/master/services/nginx/nginx.conf.template#L1

[stefanv]

Incorporated into 5ebaad88bae4dbf7cc75ea526da420d18f956dc4