Set the content type header on build

[ekohl]

Prior to this patch, the application/json Content-Type header was set but he body is a tar file as returned by Docker::Util.create_tar. That this code was there for a long time suggests Docker doesn't check the content type. However, podman's REST API does check the header and rejects the request. After this patch, it is accepted.

[ekohl]

It appears to fail on a lack of test coverage? Not sure how to solve that.

[trevor-vaughan]

@ekohl Sorry, I didn't see this before going bananas on #569. Are you OK closing this in favor of that PR?

[ekohl]

I think this is still correct. The content type is always application/json and that should be set. Retrying on a situation we already know exists is inefficient. So IMHO this should be merged. I don't like your solution to be honest.

[trevor-vaughan]

@ekohl If you try more cases, you'll find that application/json is not always the content type. I did what you did at the first go but it rapidly failed when trying other operations. The "correct" fix is to have each operation actually pass what it needs but that will take more time than I have at the moment (particularly if they're not taking patches).

[ekohl]

I agree. Given this hasn't seen a review from a maintainer in a few months I must admit I mostly forgot about this.