Docker tries to mount root directories

[frafra]

Some containers try mounting paths such as /var/log or /opt/local. This assumes that the Docker daemon is running as root, which is not always the case.

My suggestion is to drop /var/log/headstart entirely, since logs should be written to stderr/stdout in containers and then gathered by Docker, while /opt/local/renv/cache does not seem to be needed after the container has been built.