I didn’t need to use Portainer. Install the COMMUNITY SSH & Web Terminal add-on (the one made by Frenk), then use docker container ls to list docker containers. Then I accessed the required container (for me it was InfluxDB) using docker exec -it 21332546_influxdb influx -username XXX -password XXX replacing 21332546_influxdb with container ID. Not sure what you would replace influx with in your situation.
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