[x] Implement prosody update check. docker exec check_prosody_update is supposed to be called by cron;
[x] Implement _trigger_buildtag (bash script). It triggers automated docker build by webhook; The webhook URL needs to be changed;
[x] Adapt dockerhub-webhook-listener to get build notifications form docker hub, and launches a bash script to pull the updated image(s) to the host machine.
[x] Test wheter it updates on Docker Hub and re-builds on the dev host.
docker exec check_prosody_updateis supposed to be called by cron;