Remove image lookups from acorn ps

[g-linville]

In a previous PR, I changed acorn ps so that it would output the short digest instead of the tag if the tag on an app's image got moved to a newer version of the image. This resulted in a massive performance hit and needed to be undone.

I spoke with Craig and Darren, and they are okay with having the image digest as a separate column in the output so that users can differentiate between apps running different versions of an Acorn image that are using the same tag.

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• Commits follow contributing guidance
• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)