chore: abstract image pulling (backport #1137)

[mergify]

This is an automatic backport of pull request #1137 done by Mergify.
Cherry-pick of 4c0075f has failed:

On branch mergify/bp/7.x/pr-1137
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 4c0075fb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   e2e/_suites/fleet/ingest_manager_test.go
	both modified:   internal/docker/docker.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #1138 updated

• Start Time: 2021-05-06T06:41:43.272+0000

• Duration: 19 min 5 sec

• Commit: e0c4009

Test stats 🧪

Test	Results
Failed	0
Passed	152
Skipped	0
Total	152

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	152
Skipped	0
Total	152

[mdelapenya]

Needs #1140 to be merged first

[mergify]

This pull request is now in conflict. Could you fix it @mergify[bot]? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/7.x/pr-1137 upstream/mergify/bp/7.x/pr-1137
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-1137

[mdelapenya]

@Mergifyio refresh

[mergify]

Command refresh: success

Pull request refreshed