Have a look at: https://stackoverflow.com/questions/23436613/how-can-i-conve...

Or "slim": https://news.ycombinator.com/item?id=20182141

Typically, a docker filesystem doesn't include a proper unit (docker will inject "tiny init") - so you would probably have to add a kernel, and init, somehow.

I think I'd prefer to just create a vm, rather than re-use the pre-built docker.

The linked article contains a script building the file system.

It creates a block device using `qemu-img`, adds an empty filesystem using `mkfs.ext4` and then simply mounts it and copies in the files.

The previous posts cover this topic in more detail:

Day 43: Building VM images - https://jvns.ca/blog/2021/01/21/day-43--building-vm-images/

Day 44: Building my VMs with Docker - https://jvns.ca/blog/2021/01/22/day-44--got-some-vms-to-star...

It may not be what fly.io does but there's a walkthrough of the basic idea here https://iximiuz.com/en/posts/from-docker-container-to-bootab...

It's not a big secret or anything but it's changed recently and Jerome would do a better job of describing it than I would; apart from the filesystem stuff, we have an init we wrote in Rust that does a bunch of the lifting.

We use containerd with the devmapper snapshotter! Works nicely.

We create a hard link to the resulting device for the root drive inside firecracker.

I’d love to know more about this as well. That’s the part usually nobody talks about.