Fixes issue with building Wireguard package withdocker on DSM 7.0 #88

[bb-Ricardo]

The build env extracts an own set of "/dev" inside the chroot and somehow the container is not able to write to /dev/null or read from other devices. This patch adds a bind mount to mount the container "/dev" directory into the chroot environment. Then the build works as expected.

[OrpheeGT]

Just tested it and confirm it works ! It also does not break anything on standard Linux (Ubuntu) system.