All your statics could be generated during the "build" phase. To do so, you'll need to include your config.php file in your repository, with all scopes and themes.
If you don't have any config.php file in your app/etc folder, then it will be generated during the build phase automatically.
As a reminder, you can extract all these information from your current environnement to your config.php file with this command:
bin/magento app:config:dump scopes themes
During the build phase the database is not available. Regarding this, we added the variable MAGENTO_CONFIG_SET_VALUES in order to give context to the content static deploy then statics are full generated.
Feel free to use following mysql request to find which config needs to be filled in:
SELECT value from core_config_data
where path REGEXP ('^dev/(js|css)/*')