Update buildAndMaybePush to use more robust method to find local digests

[IRCody]

Older versions of docker sometimes will report the digest as "" which would break with the previous implementation. This implementation should correctly grab the values.

Fixes #[insert GH or internal issue link(s)].

Describe your change...

---

• Tests
  • [ ] TODO (before merge)
  • [x] Tests included
  • [ ] Tests not necessary because:
• Wrangler / Vite E2E Tests CI Job required? (Use "e2e" label or ask maintainer to run separately)
  • [ ] I don't know
  • [ ] Required
  • [x] Not required because: Modifying existing behavior that is not e2e tested afaik
• Public documentation
  • [ ] TODO (before merge)
  • [ ] Cloudflare docs PR(s):
  • [x] Documentation not necessary because: Makes existing method more robust but doesn't change user ux
• Wrangler V3 Backport
  • [ ] TODO (before merge)
  • [ ] Wrangler PR:
  • [x] Not necessary because: feature doesn't exist in v3

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 0bf56245c6ea23e908797420727485cae18a74f0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
wrangler	Patch
@cloudflare/vitest-pool-workers	Patch

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[pkg-pr-new[bot]]

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commit: 0bf5624

[CarmenPopoviciu]

left a few nit comments, but LGTM otherwise.

[github-actions[bot]]

Failed to automatically backport this PR's changes to Wrangler v3. Please manually create a PR targeting the v3-maintenance branch with your changes. Thank you for helping us keep Wrangler v3 supported!

Depending on your changes, running git rebase --onto v3-maintenance main cdr/docker-inspect might be a good starting point.

Notes:

• your PR branch should be named v3-backport-9641
• add the skip-v3-pr label to the current PR to stop this workflow from failing