Lambda docker image

[nnovaeshc]

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Fixes #

Proposed Changes

• Add a new variable (lambda_image_uri) and logic to decide beteen using an externally provided lambda or the builtin lambda code.

Description

This should address #61

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).