It appears that adding the following package rule to renovate.json makes Dockerfile node image follow LTS versions of node even if there is no explicit mention of it.
Without this, the node version is updated to latest version instead of LTS.
Additionally, renovate does not support docker image suffixes yet. See this.
To support image names like node:18.0-alpine, we use versionCompatibility to define how the version with suffix should be parsed.
The versionCompatibility is not necessarily needed if the image version does not use any suffix.
"packageRules": [
{
"matchDatasources": ["docker"],
"matchPackageNames": ["node"],
"versionCompatibility": "^(?<version>[^-]+)(?<compatibility>-.*)?$",
"versioning": "node"
}
]