Building firecracker from the source requires docker as a dependency. Documentation pages(link) says
Firecracker uses a Docker container to standardize the build process.
And while building firecracker from the source, it pulls fcuvm image from dockerhub.
How is firecracker connected to docker and what exactly is this docker image doing?
Every product needs some environment to test and build its logic. Similarly, firecracker also needs some libraries to test and build. Firecracker maintainers created a docker image "fcuvm" and hosted it in the AWS ECR. Firecracker has a CI pipeline for testing. This pipeline uses the "fcuvm" image to create the environment and test the logic.
You can run the testing in your local machine also using "fcuvm" image. Firecracker devtool will make your task easy.