Renovate Docker image is missing "flutter" tool

[Sander0542]

How are you running Renovate?

Self-hosted Renovate

If you're self-hosting Renovate, tell us which platform (GitHub, GitLab, etc) and which version of Renovate.

GitHub

Please tell us more about your question or problem

The Flutter (and Dart) CLI tools are missing in the Renovate Docker image. I am receiving the following comment on my Renovate pull requests.

This issue looks similar to #2602

Logs (if relevant)

Logs

⚠️ Artifact update problem
Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

any of the package files in this branch needs updating, or
the branch becomes conflicted, or
you click the rebase/retry checkbox if found above, or
you rename this PR's title to start with "rebase!" to trigger it manually
The artifact failure details are included below:

File name: pubspec.lock
Command failed: flutter pub upgrade events_emitter
/bin/sh: 1: flutter: not found

[rarkins]

They need to be added to https://github.com/renovatebot/base-image/blob/main/Dockerfile (different repo). PR welcome

[Sander0542]

I created the following PR, but I have to check whether is works.

renovatebot/base-image#1448