Sign all container images

[Demi-Marie]

All container images should be signed, and the commands that download them need to check the signatures.

[fgimenez]

We are using several external dependencies in the form of helm charts, each of them with its own image(s). We could try to do it for our images, namely the default docker parity/polkadot, not sure if it is being signed now. If so, we would need to make the signing public key available to the k8s worker nodes and define a docker policy in each of them. I'll check if this is possible in the managed kubernetes we are using at the moment and post back here.