The docker build target must be gitlab_webhook for local gitlab install

[ocroz]

Thanks to #682, I could fix my local GitLab installation with this docker build command:

docker build . -t pr-agent:4ab747d_gitlab --target gitlab_webhook -f docker/Dockerfile

The target must be gitlab_webhook when building the Docker image.
However, I cannot find this information in the current documentation anymore.
Could you please re-add this important information?

[mrT23]

welcome to open a PR to update the relevant docs