The scheduler in ephemerd is event-driven. GitHub sends a workflow_job webhook when a job is queued, when it starts and when it completes. ephemerd creates and destroys runners to match. For a while that worked well enough that I stopped thinking about it. Then a docker-build job failed with “The self-hosted runner lost communication with the server”. I pulled the node’s journal. ephemerd had destroyed that runner itself, on purpose, while it was running the build.

GitHub Actions has a container: key that runs a job’s steps inside an image of your choosing. It’s popular because it gives you a reproducible toolchain without building a custom runner image. On a self-hosted runner it works by having the runner drive Docker. On ephemerd, “Docker” is a shim that speaks the Docker API and creates containers in containerd. Getting bind mounts right through that shim took four PRs in a week. A couple of months later the version that worked…

A few shipmates bugs don’t need a whole post each. They were too good to leave in commit messages, though.

I have CI that needs Linux amd64, Linux arm64, Windows and macOS. GitHub-hosted runners for all four get expensive fast, especially the arm64 and Mac minutes. Self-hosted runners fix the bill and create a different problem. A stock actions/runner on a box you own runs whatever the workflow says. It keeps whatever the last job left behind. And it’s one fork PR away from somebody running code on your LAN.

I’ve been building ephemerd, a daemon that runs self-hosted GitHub Actions runners with a fresh environment per job. On a Windows host, Windows jobs run in Hyper-V isolated containers. Linux jobs still need a Linux kernel from somewhere. The obvious answer on Windows in 2026 is WSL2, and that’s what I used first. It worked great from my terminal and fell over the moment ephemerd became a real Windows service.