[SalesForce] sfdx scratch org Partner Community User license

Probably a bit late but better than never.

You can use sfdx force:mdapi:deploy -d data/standard-object-stub/ -w 1 -u $SCRATCH_ORG_ALIAS with a package.xml that references a Case.object file with the following content in a objects subfolder before you deploy your sources

<?xml version="1.0" encoding="UTF-8"?>
<CustomObject xmlns="http://soap.sforce.com/2006/04/metadata">
    <sharingModel>Read</sharingModel>
</CustomObject>

This will only change the sharing setting, which will take some time so you might want to delay the further steps like package installation or source push a bit.

Update 12/2019:

Within the topic of unlocked packaged (that might have managed packages with certain OWD sharing settings as a dependency) I've come across a probably interesting known issue in this field:

success.salesforce.com/issues_view?id=a1p3A000001SGtdQAG

That known issue raises hopes on better support for changing OWD sharing settings based on scratch org definition files.

Update 09/2020:

Actually that got improved recently, you can use the scratch org definition file:

{
  "orgName": "Scratch Org",
  "edition": "Developer",
  "language":"en_US",
  "features": [
    "AuthorApex",
    "ContactsToMultipleAccounts",
    "StateAndCountryPicklist"
  ],
  "settings": {
    "accountSettings": {
      "enableAccountTeams": true
    }
  },
  "objectSettings": {
    "opportunity": {
      "sharingModel": "private"
    },
    "account": {
      "sharingModel": "read"
    },
    "case": {
      "sharingModel": "read"
    },
    "contact": {
      "sharingModel": "private"
    }
  }
}

Use a Developer edition scratch org

{
  "orgName": "My Dev Scratch Org",
  "description": "",
  "language": "en_US",
  "edition": "Developer",
  ....
}

You will get the following licences

|          License Type         | Number of licenses |
|:-----------------------------:|--------------------|
| Customer Community            | 5                  |
| Customer Community Login      | 5                  |
| Customer Community Plus       | 5                  |
| Customer Community Plus Login | 5                  |
| Partner Community             | 5                  |
| Partner Community Login       | 5                  |