There is currently no way to enable "Contacts to Multiple Accounts" in the scratch org configuration file: https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_scratch_orgs_def_file_config_values.htm
Our repository/dx project includes the AccountContactRelation object and references it in layouts which means that this setting must be manually enable for every new scratch org before the source can be pushed to the scratch org. Because of this , we are not able to setup or CI pipeline to create temporary scratch orgs for testing.
Are there any workarounds for this? Also, is access to this setting in the config file planned to be released anytime soon?