docs(gitlab): add hostType to registry hostRules guide

[MalteMagnussen]

Fix docs about private GitLab Container Registry config.

Changes

Update Docs about GitLab private registry, so that the example works.

Context

Wasted 4 hours trying to get Renovate to work with a private GitLab registry, and I never once considered that the example might be wrong, before I stumbled on this issue:

• Use GitLab platform token for container registry #17940

If you use env to config host rules, then you need to config like this:

• DOCKER_GITLAB_EXAMPLE_COM_PASSWORD=renovate-bot
• DOCKER_GITLAB_EXAMPLE_COM_USERNAME=glpat-xxxxxxxx

And if you use "normal" json config host rules, they need to look like this:

  "hostRules": [
    {
      "hostType": "docker",
      "matchHost": "gitlab.example.com",
      "username": "renovate-bot",
      "password": "glpat-xxxxxxxxxx"
    }
  ],

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 35.75.0 🎉

The release is available on:

• GitHub release
• 35.75.0

Your semantic-release bot 📦🚀

[jullle3]

@MalteMagnussen 💪