1. Authenticate with a DevHub
2. Run sfdx force:org:create -f config/project-scratch-def.json -a custom-object-issue
3. Run sfdx force:source:push -u custom-object-issue
4. Run sfdx force:org:open -u custom-object-issue -p lightning/setup/SecuritySharing/home

Notice that even though Example_Object__c specifies externalSharingModel of ReadWrite, the object is actually deployed with the sharing set to Private.