If you want to test the effects of switching to the new build image on your current site, you could try creating a new site with the same code as your production site and uses the Focal build image. Then you can do some testing to make sure it deploys before you switch the production site.
I’m the co-maintainer for a public open source project where the owner is missing. Can I configure the build image from the netlify.toml? I don’t have access to the Site Settings for the project, but I have complete control of netlify.toml.
Unfortunately, this is not configurable in code - we do need the netlify site owner to make the change in our UI or via our API.
When you say they are “missing” - what do you mean exactly? We’ll certainly have their netlify login email and can contact them on your behalf, to ask for their help (or ask if they are willing to delegate ownership of the netlify site to you), if you’d like to share what site you are working on?