Use the official Lambda Docker images from AWS

[jsf9k]

🗣 Description

This pull request modifies the repo to use the official Lambda Docker images from AWS.

💭 Motivation and context

In addition to it just being a better idea in general to use the official images from AWS, the official images are still being updated. The lambci/lambda Docker images we were using have not been updated in years.

🧪 Testing

I deployed new Lambda functions built using these changes and verified that they function as expected.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.

✅ Post-merge checklist

• Create a release.