How can I override the default container image?

As you can see above you can override any step in any build pack; but you can also override the container image used by default in all the steps by adding this YAML to your jenkins-x.yml :

pipelineConfig:
  agent:
    label: jenkins-go
    container: somerepo/my-container-image:1.2.3

You can see the effect of this change locally before you commit it to git via the jx step syntax effective command:

jx step syntax effective -s

For more detail check out how to override steps