Ah, thank you so much. I should have checked this.
The html file is rendering fine but my png file isn't being created by webshot because PhantomJS is not found.
I have got it working now.
2020-04-09T13:16:49.571637+00:00 shinyapps[2065231]: Output created: Summary.html
2020-04-09T13:16:49.622275+00:00 shinyapps[2065231]: PhantomJS not found. You can install it with webshot::install_phantomjs(). If it is installed, please make sure the phantomjs executable can be found via the PATH variable.