fix broken docker build
fix broken docker build

Current docker build output: /bin/sh: 1: locale-gen: not found
Locale-gen is not provided anymore by ubuntu base image, so it must be installed beforehand.
strukturag/spreed-webrtc,strukturag/spreed-webrtc,strukturag/spreed-webrtc,strukturag/spreed-webrtc,strukturag/spreed-webrtc,strukturag/spreed-webrtc