[Actions] Document authentication for service containers

[ndench]

Document how to authenticate with private registries and DockerHub when pulling images for service containers.

Why:

The documentation for service containers doesn't currently show how to specify credentials.
However, there's a blog post from 2020 showing how to do it.

Closes: #29914

What's being changed (if available, include any code snippets, screenshots, or gifs):

The docs don't show how to specify credentials when using service containers. Credentials may be necessary to pull images from private registries or to authenticate with DockerHub in order to increase rate limits.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline.

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/using-containerized-services/about-service-containers.md	fpt ghec ghes@ 3.11 3.10 3.9 3.8	fpt ghec ghes@ 3.11 3.10 3.9 3.8

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[cmwilson21]

@ndench Thank you for opening a PR and linking it to your issue! ✨

I'll get this triaged for review! ⚡

[hubwriter]

@ndench - Great change, many thanks for adding this! 👍

Just one suggestion - which you can click to commit.

[hubwriter]

Merging this now

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡