add: user directive in Acornfile (e.g. `user: "1000:1000"`) (#718)

[iwilltry42]

This translates to containerSecurityContext.RunAsUser|RunAsGroup

Ref #718

Example:

containers: foo: {
  image: "some/image:v1"
  user: "1000:1000"
}

Checklist

• [x] The title of this PR would make a good line in Acorn's Release Note's Changelog
• [x] 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
• [x] All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• [x] Commits follow contributing guidance
• [x] Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• [x] 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)