Use docker 1.10 user namespacing

[vektah]

Docker 1.10 introduces user namespacing. This allows us to lock down the docker daemon to a non root user account.

This means doing

docker run -it -v /:/hostroot busybox touch /hostroot/things-owned-by-root

Will no longer be able to modify the host as root.

Apart from being much more secure, it means that files created inside a docker based build script will be owned by buildkite-agent, instead of root. This happens often when projects volume mount the project root into the container.

This is a brand new feature, and there are likely to be bugs. At the very least wait till 1.10.3 for moby/moby#20446 to be fixed.

Things that do not work with userns:

Using --privileged mode flag on docker run

• sharing PID or NET namespaces with the host (--pid=host or --net=host)
• sharing a network namespace with an existing container (--net=container:other)
• sharing an IPC namespace with an existing container (--ipc=container:other)
• A --readonly container filesystem (this is a Linux kernel restriction against remounting with modified flags of a currently mounted filesystem when inside a user namespace)
• external (volume or graph) drivers which are unaware/incapable of using daemon user mappings

[mbj]

This PR was merged with an empty diff? intentional?