Choose JNLP or SSH to launch your slave.

JNLP
Make sure container can access Jenkins master. Take image jenkinsci/jnlp-slave 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 jenkinsci/ssh-slave as a reference

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