How to start Splunk Forwarder within a script
The arguments you are looking for are --accept-license --answer-yes --no-prompt.
A Splunk employee publishes some Docker images on his own time that can cater for this already. Have a look at SPLUNK_START_ARGS in the forwarders entrypoint.sh.
Also, Docker natively supports sending its own logs to Splunk. This includes the stdout and stderr of all containers so you may not need your own forwarder.