Print a warning when system has low entropy

[CrimsonGlory]

Given that if the system has low entropy (cat /proc/sys/kernel/random/entropy_avail returns a number less than 100) docker-compose commands hang for several minutes (even with simple commands like docker-compose --version or docker-compose --help), a warning should be printed:
WARNING: Very low entropy. docker-compose commands can hang.

This would have avoided lots of wasted time.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[CrimsonGlory]

(This issue was for version 1.x. Probably it doesn't apply to 2.x so I'll close)