So I haven't tested this specific example (but it should work):

"packageRules": [
    {
      "matchDatasources": ["docker"],
      "matchPackageNames": ["ghcr.io/netbootxyz/netbootxyz"],
      "versioning": "regex:^(?<major>\\d+)\\.(?<minor>\\d+)\\.(?<patch>\\d+)-nbxyz(?<build>\\d+)$"
    }
  ]

Netboot.xyz has its docker images tagged in a non-standard way necessitating a custom package rule so that renovate bot can properly pick up updates. I thought this went into the config.js that renovate bot uses for its own configuration but no it goes in your repo's renovate.json file like so:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "extends": [
    "config:recommended"
  ],
  "packageRules": [
    {
      "matchDatasources": ["docker"],
      "matchPackageNames": ["ghcr.io/netbootxyz/netbootxyz"],
      "versioning": "regex:^(?<major>\\d+)\\.(?<minor>\\d+)\\.(?<patch>\\d+)-nbxyz(?<build>\\d+)$"
    }
  ],
  "assignees": [
    "UntouchedWagons"
  ],
  "reviewers": [
    "UntouchedWagons"
  ]
}