What do FROM nginx and COPY index.html /usr/share/nginx/html/ establish?
FROM nginx
COPY index.html /usr/share/nginx/html/
They install the Nano editor in the Nginx image.
They use Nginx as the base image and place the custom page in its web root.
They start a container and publish its web port on the host.
They retrieve Nginx's default page from a running container to the host.