2.0.3 - cosign auth fix for container-image signing

container-image template: cosign now does an explicit registry
login before signing the pushed image. buildah and cosign use
different auth conventions (podman auth.json vs docker
config.json), so the buildah login from before_script was not
visible to cosign. Bairn v0.5.0 hit this; the cli image was
pushed unsigned.

No template inputs change. Consumers on 2.0.2 can bump to 2.0.3
with no other edits.