Add CI to test on read-only file systems

[atomb]

PR #139 adds an option that should disable any file creation and allow argo-based servers to run on read-only file systems. However, it's possible that we've missed file creation, or that we could add some in the future. Testing this will require some significant infrastructure, but I suspect that creating a Docker image containing file-echo-api and running it with docker run --read-only will probably get us pretty far.