renovate/lib/modules/manager/dockerfile/readme.md

Extracts image references in a Dockerfile and/or Containerfile.

Renovate's managers does not understand versioning, that's up to Renovate's versioning modules. The default docker versioning for container image datasources treats suffixes as "compatibility", for example: -alpine. Many container images are not SemVer compliant because they use such suffixes in their tags.

If Renovate does not update your container images correctly, you may need to tell Renovate what versioning it should use. For example, if you know that an image follows SemVer, you can tell Renovate to use "semver" versioning for that image:

{
  "packageRules": [
    {
      "matchDatasources": ["docker"],
      "matchPackageNames": ["whitesource/renovate"],
      "versioning": "semver"
    }
  ]
}

Read Renovate's Docker Versioning docs to learn more.