Put this in a script.sh somewhere, and add it to your cron.
#!/bin/bash
CONTAINER=fireflyiii/data-importer:develop-cli
docker pull $CONTAINER
docker run \
--rm \
-v $SCRIPT_DIR/json-files:/import \
-e FIREFLY_III_ACCESS_TOKEN=... \ # change me!
-e FIREFLY_III_URL=.... \ # change me!
-e VANITY_URL=.... \ # change me!
# optional settings for email:
-e ENABLE_MAIL_REPORT=true \
-e MAIL_MAILER=smtp \
-e [email protected] \
-e MAIL_USERNAME==... \
-e MAIL_PASSWORD==... \
-e MAIL_FROM_ADDRESS==... \
-e MAIL_DRIVER=smtp \
-e MAIL_HOST==... \
-e MAIL_PORT=587 \
-e MAIL_ENCRYPTION=tls \
# import settings
-e IGNORE_DUPLICATE_ERRORS=true \
-e IGNORE_NOT_FOUND_TRANSACTIONS=true \
-e IMPORT_DIR_ALLOWLIST=/import \
-e LOG_LEVEL=debug \
-e PHP_OPCACHE_ENABLE=1 \
# auth settings for GoCardless
-e NORDIGEN_ID=... \
-e NORDIGEN_KEY=... \
$CONTAINER