BUG: end-to-end broken for docker-based deployment

[thomas-fossati]

Reported by @gkostal on v0.0.2412

The provisioning step fails due to a problem with propagating the correct media type.

To repro:

git checkout v0.0.2412
make really-clean
make docker-deploy
end-to-end/end-to-end-docker provision

The provisioning step will fail with a 500 from the veraison API.

(Note that internal server error is not the right status to return in this situation. This is tracked separately by #283)