docker workflow: support submodules initialisation

[gtauzin]

Description

This ensures that the docker.yml action runs on repos that rely on git submodule.

Changes Made

We enable setting the checkout submodules by introducing an action input initialize-submodules.
This was tested successfully on a private repository.

Checklist

• I have used a PR title that is descriptive enough for a release note.
• I have tested these changes locally.
• I have added appropriate tests or updated existing tests.
• I have tested these changes on a cluster [name of the cluster] / customer [name of the customer]
• I have added appropriate documentation or updated existing documentation.