How to run Nginx within a Docker container without halting?

nginx, like all well-behaved programs, can be configured not to self-daemonize.

Use the daemon off configuration directive described in http://wiki.nginx.org/CoreModule.

To expand on Charles Duffy's answer, Nginx uses the daemon off directive to run in the foreground. If it's inconvenient to put this in the configuration file, we can specify it directly on the command line. This makes it easy to run in debug mode (foreground) and directly switch to running in production mode (background) by changing command line args.

To run in foreground:

nginx -g 'daemon off;'

To run in background:

nginx

To expand on John's answer you can also use the Dockerfile CMD command as following (in case you want it to self start without additional args)

CMD ["nginx", "-g", "daemon off;"]

Just FYI, as of today (22 October 2019) official Nginx docker images all have line:

CMD ["nginx", "-g", "daemon off;"]

e.g. https://github.com/nginxinc/docker-nginx/blob/23a990403d6dbe102bf2c72ab2f6a239e940e3c3/mainline/alpine/Dockerfile#L117