Add s390x support for kube-webhook-certgen docker image

[sid226]

Error while pulling image on s390x machine.

Failed to pull image "jettech/kube-webhook-certgen:v1.2.0": rpc error: code = Unknown desc = no matching manifest for linux/s390x in the manifest list entries

[vsliouniaev]

@sid226 Could you check if v1.2.2 works for you?

[sid226]

@vsliouniaev Yes its working. Thanks.
is it possible to add for 1.2.1 as well?

[vsliouniaev]

I don't want to alter the existing docker manifest for anyone who may be already using it.

There are no code changes between 1.2.1 and 1.2.2, just an amended CI process to build an image for the target architecture.