ci: publish docker image only to public registry

[ggera]

fixes KILTProtocol/ticket#3600

Maintain only two versions of docker images on the public registry.
kiltprotocol/kilt-node
kiltprotocol/standalone-node

Metadata Diff to Develop Branch

Peregrine Diff

Spiritnet Diff

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here