Maintainance page while Docker container is set up

[saitho]

Idea: When a Docker container is set up (either created or removed before recreation) a maintainance page should be visible.

Maybe we can generate a file in the project directory ENABLE_MAINTAINANCE.
The webserver plugin would check if that file is present and show the StackHead maintainance page unless the file is removed again.

The maintainance page is a static HTML page. There should be a default page by StackHead that includes all resources inline (i.e. CSS, images as SVG) so no external resources need to be loaded. The user is able to set their own maintainance page HTML file during deployment or in server setup.

Nginx:

• https://gist.github.com/bcambel/6247835
• https://serverfault.com/questions/577370/how-can-i-use-environment-variables-in-nginx-conf