Often times tools just want to display static HTML from ~/public_html/ and that's it. The best way to do that right now is to use the php56/php72 webservice types. However, when it comes to upgrades (new OS/PHP version), we require user intervention to upgrade because we don't know that the tool is just static HTML and not actually using PHP.
Having a static HTML webservice type signals to Toolforge admins that it can be safely upgraded to the new version without user intervention.
For now, "static-html" can be an alias for php72, we don't necessarily need a separate container for it.