Use configured GitlabToken for Docker datasource authorization

[secustor]

What would you like Renovate to be able to do?
We are using a self hosted instance of Gitlab and Renovate.
The docker registry provided by Gitlab is served under another domain ( e.g. gitlab.example.com vs registry.example.com )
Because the registry is served by Gitlab, the Gitlab token configured by gitlab_api_token should be reused for authentication.

Did you already have any implementation ideas?
self hosted configuration:

{
  "endpoint": "https://gitlab.example.com/api/v4/",
  "platform": "gitlab",
  "token": "xxxxxxxxx"
}

I see to implementation variants:

1. Implement a separate function in the docker data source which tries to authorize against the registry URL with the API token. This could be optional maybe.
2. add variable expansion in the host rules to insert the gitlab_api_token in the password field with:
   a. From environment variables e.g.:

{
  "hostRules": [
    {
      "domainName": "registry.example.com",
      "username": "renovate",
      "password": "${GITLAB_API_TOKEN}"
    }
  ]
}

b. From other configurations e.g.

{
  "hostRules": [
    {
      "domainName": "registry.example.com",
      "username": "renovate",
      "password": "{{config.token}}"
    }
  ]
}

[rarkins]

Can you add hostRules using the bot config (e.g. config.js) which allows variable substitution? e.g.

module.exports = {
  hostRules: [
    {
      domainName: "registry.example.com",
      username: "renovate",
      password: process.env.GITLAB_API_TOKEN}
    }
  ]
}

[secustor]

Thx that works!

But maybe a reuse of other configuration values could be helpful feature. WDYT?

[viceice]

@secustor checkout #8070

[secustor]

@viceice yeah that would fit my use case