It's been a while since I've worked with Crystal. If your rendering doesn't require an MSI to be installed, and simply uses a dll that your project references, that should work.
However, if that doesn't work, then your option would be a Web App for Container. Configure your docker image to install and configure Crystal, push the image to registry and pull the image from that registry. You can use Quickstart: Run a custom container on App Service - Azure App Service | Microsoft Learn as a guide.