My Docker file that I used to convert my Node/Express API that was running in lambda consisted of...
- setting environment variables (I had to do the same thing in my CF template for lambda)
- retrieving the Node Docker image (one line of code - I had to specify one line of code in my CF template with lambda to specify that is was using the Node runtime)
- copying the files from the directory to the container (as opposed to specifying the locsl path of the source code in the CF Template)
- Specifying the Docker Run command (as opposed to specifying the lambda handler).
- running “Docker build” as opposed to “CloudFormation package”.
How it does it is an implementation detail.
In fact, if you use lambda layers, it’s almost equivalent to building Docker images.