add sanic-nginx-docker-example to extensions.md
add sanic-nginx-docker-example to extensions.md

yunstanford/sanic,Tim-Erwin/sanic,channelcat/sanic,lixxu/sanic,jrocketfingers/sanic,lixxu/sanic,r0fls/sanic,r0fls/sanic,lixxu/sanic,ashleysommer/sanic,jrocketfingers/sanic,ashleysommer/sanic,channelcat/sanic,yunstanford/sanic,Tim-Erwin/sanic,ai0/sanic,ai0/sanic,ashleysommer/sanic,channelcat/sanic,yunstanford/sanic,lixxu/sanic,channelcat/sanic,yunstanford/sanic