Allow setting docker pull_policy via gitlab-runner register CLI
## Description
Currently, when registering a runner with the Docker executor using the `gitlab-runner register` command, there is no option to configure the Docker `pull_policy` directly via CLI parameters.
The only way to set this value is to manually modify the `config.toml` file after the runner has already been registered.
This makes it difficult to fully automate runner provisioning.
## Problem to solve
When provisioning runners automatically (for example using Docker containers, infrastructure automation, or scripts), all configuration should ideally be set during the `gitlab-runner register` process.
At the moment, the Docker pull policy can only be configured manually in `config.toml`. Example configuration:
`[runners.docker]`\
`pull_policy = ["if-not-present"]`
This requires an additional configuration step after registration, which complicates automated setups.
## Proposal
Please add a CLI parameter to the `gitlab-runner register` command that allows configuring the Docker pull policy during registration.
For example:
`gitlab-runner register `\
`--url "``https://gitlab.com/``" `\
`--registration-token "" `\
`--executor docker `\
`--docker-image docker:20.10.24 `\
`--docker-volumes /var/run/docker.sock:/var/run/docker.sock `\
`--docker-pull-policy if-not-present`
The command would then generate the corresponding entry in config.toml:
`[runners.docker]`\
`pull_policy = ["if-not-present"]`
issue