Add a `serve-nginx-docker` command

[bartfeenstra]

Add a serve-nginx command to serve a site explicitly using the Dockerized nginx server. This prepare for the upcoming removal of server negotiation.

Proposals

• Extract the Docker functionality to a new extension that requires the Nginx extension
• add the command to the Docker extension and name it serve-docker
• this new extension has a requirement that checks if Docker is available
• these proposals are BC breaks! Maybe best for a 0.4.x follow-up