I just realized, that it is simple as mounting /dev/urandom from the host as /dev/random into the container:
$ docker run -v /dev/urandom:/dev/random ...
The result is as expected:
$ docker run --rm -it -v /dev/urandom:/dev/random ubuntu dd if=/dev/random of=/dev/null bs=1 count=1024
1024+0 records in
1024+0 records out
1024 bytes (1.0 kB) copied, 0.00223239 s, 459 kB/s
At least I know how to build my own boot2docker images now 😉