fix(cd): use correct sha for PR based docker build

[fffonion]

KAG-3251

Summary

use github.event.pull_request.head.sha instead of github.sha on a PR, as github.sha on PR is the merged commit (temporary commit).

also correctly set the KONG_VERSION env var.

Checklist

• The Pull Request has tests
• [na] A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• [na] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

[fffonion]

docker inspect kong/kong:b32b41d93f1e4001ee16006ddca8561021aeedd6-ubuntu|jq .[].Config.Labels
{
  "maintainer": "Kong Docker Maintainers <[email protected]> (@team-gateway-bot)",
  "org.opencontainers.image.created": "2023-11-29T07:43:19.673Z",
  "org.opencontainers.image.description": "🦍 The Cloud-Native API Gateway",
  "org.opencontainers.image.licenses": "Apache-2.0",
  "org.opencontainers.image.ref.name": "ubuntu",
  "org.opencontainers.image.revision": "b32b41d93f1e4001ee16006ddca8561021aeedd6",
  "org.opencontainers.image.source": "https://github.com/Kong/kong",
  "org.opencontainers.image.title": "kong",
  "org.opencontainers.image.url": "https://github.com/Kong/kong",
  "org.opencontainers.image.version": "b32b41d93f1e4001ee16006ddca8561021aeedd6-ubuntu"
}