Choose JNLP or SSH to launch your agent.

JNLP
Make sure container can access Jenkins master. Take image jenkins/inbound-agent as a reference

SSH
Make sure container initialize with a SSH server and Jenkins can access to container
You should install Jenkins master in the same ACS cluster.
Take image jenkins/ssh-agent as a reference

If using SSH, here is a sample of arguments:
 Docker image: jenkins/ssh-agent
 Command:
 Arguments:
 Environment Variable: Key: JENKINS_AGENT_SSH_PUBKEY, Value: Your public key
 Launch Method: SSH