Add docker plugin config option image_pull_timeout value for default timeout

[allisonlarson]

Description

Adds a docker plugin config option for setting a default image_pull_timeout for all tasks. This can be overridden by the task config option, if a different timeout is desired. The default timeout remains at 5m.

Also adds the value to the docs.

Testing & Reproduction steps

Links

Fixes #25057

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[aimeeu]

Thanks for updating the docs! I left a nit style guide suggestion. Nomad content has a lot of future tense, but I'm slowly changing that to present tense. I put the most important style guidance into our team-nomad docs.md file so you don't have to wade through the existing Education and Eng style guides.

[tgross]

LGTM!

Because this is a user-facing change we want a changelog entry. There's a makefile target make cl which runs a little "wizard" in the shell to generate a changelog snippet; all the snippets for a given version get combined with automation during the release process.

[tgross]

LGTM!

I've removed the backport/website label because we don't want docs changes for unreleased new features to ship immediately. Edit: and the backport/1.9.x label as the next scheduled release is the 1.10 GA so this, as a new feature, will ship in 1.10 but not 1.9.x

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.