dcinja example

Here is an example for building a nginx config file when build docker. Once the docker image is ready. The config will not be changed.

1. Prepare a Dockerfile, ref the folder example.
2. Create an ARG, and build docker with --build-arg. For the example, the command line would be docker build . --tag dcinja_example --build-arg NAME=YOYO.
3. Run the docker example, docker run --rm -p 8080:8080 -it dcinja_example.
4. Launch browser and browse http://127.0.0.1:8080
5. Open "Developer Tools" > "Network Inspector", check the request header "X-Dcinja", it should include the building arg.