Docker container based on official image blocks during startup in VMs

[marthjod]

The Docker image on https://hub.docker.com/r/spreed/webrtc/ still contains the entrypoint script using /dev/random (https://github.com/strukturag/spreed-webrtc/commit/172ac704e3c8dfd63f34c8b0221dd630b295aabb), causing containers to block forever during startup in environments lacking randomness (VMs, for instance).

Reproduce:

docker run -d ... spreed/webrtc
...
Creating new server secrets ...
# (blocking)