$ mkdir -p A/dev/foo
$ mkdir -p B/dev/bar
$ cat > Dockerfile <<EOF
from shykes/gordon
ADD A /
ADD B /
EOF
$ docker build -t failtest .
[...]

The contents of the image should be:

/dev/foo
/dev/bar

Instead they will be:

/dev/foo
/dev/dev/bar

This is because of the behavior of cp -ar which changes when the destination directory already exists.
It should be replaced by the more reliable method of tar/untar. This method is also used in the rest
of the codebase.