Update Content-Type behavior

[markdboyd]

Changes Proposed

Related to #81

The discussion in #81 highlights a potential bug in this proxy: it always adds a default Content-Type header to the response, even in cases like HTTP 204/304 responses which have no response body and thus for which a Content-Type header is inappropriate.

This PR updates the Nginx configuration to only set a default Content-Type header when the response status code is not 204 or 304.

The PR also adds some Docker configuration for running OpenResty with a basic Node app to make local testing much easier.

Security Considerations

It seems like adding a Content-Type header was done to resolve a POAM: cloud-gov/product#540

At the same time, it seems like the Content-Type header itself may have been an afterthought: #6

But for 204/304 responses where no response body is expected, I don't see how adding a Content-Type header is ever appropriate.