docs: add Docker installation instructions

[uzairhameed]

Adds a Docker section to the install page for v3.6, resolving the gap between the GitHub release notes and the official documentation.

The GitHub release notes (e.g. https://github.com/etcd-io/etcd/releases/tag/v3.6.12) currently include installation instructions for Linux, macOS, and Docker. The official install page (https://etcd.io/docs/v3.6/install/) already covers Linux and macOS, but was missing Docker instructions entirely.

As part of the effort to move installation instructions out of the release notes and into the official docs (#1174), this PR adds the missing Docker section.

Changes:

• Added a new ## Docker section to content/en/docs/v3.6/install.md
• Includes instructions for running etcd via Docker using
  gcr.io/etcd-development/etcd (primary) and quay.io/coreos/etcd (secondary)
• Uses {{< param git_version_tag >}} to keep the version reference
  consistent with the rest of the page

Closes #1172

[k8s-ci-robot]

Hi @uzairhameed. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[ivanvc]

Hi, @uzairhameed. Thanks for the pull request. Please ensure your commit is signed so the developer certificate of origin (DCO) check passes, i.e:

git rebase HEAD~1 --signoff
git push --force

[uzairhameed]

Hi, @uzairhameed. Thanks for the pull request. Please ensure your commit is signed so the developer certificate of origin (DCO) check passes, i.e:

git rebase HEAD~1 --signoff
git push --force

@ivanvc I have signed the commit. Please let me know if anything else is needed.

[ivanvc]

/ok-to-test

[ivanvc]

@uzairhameed, to make the linter pass, I would suggest not touching anything but the fragment you're adding.

Please also add this section for 3.5 and 3.7. Thanks :)

[uzairhameed]

/retest

[uzairhameed]

/retest

[uzairhameed]

@ivanvc I have added the Docker section to 3.5 and v3.7 and tests passed. Please let me know if anything else is needed.

[ivanvc]

LGTM. Thanks, @uzairhameed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ivanvc, uzairhameed

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