Use /dev/urandom to generate random images
Use /dev/urandom to generate random images

The existing code read a relatively large number of bytes from
/dev/random. Since /dev/random blocks until entropy is available, this
could hang on systems like VMs that don't have as many sources of
entropy. Using /dev/urandom is much more reliable.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

docker/golem,docker/golem