Update Docker workflow to build and push to Docker Hub.

FFmpeg 4.4 (alfg#95)

* FFmpeg 4.4

* upgrade: ffmpeg 4.4 for cuda image and update readme.

Set Alpine:3.13 for ffmpeg-build and release image as well.

Upgrade NGINX to 1.18.0 and FFmpeg to 4.3.1. (alfg#71)

* Upgrade NGINX to 1.18.0 and FFmpeg to 4.3.1.

* Fix readme.

Add /etc/nginx files to release image before copying over the nginx t…

…emplate.