fixed Dockerfile
fixed Dockerfile

docker image couldn't be built because libxml and libxslt were missing
for building lxml

SDNTB-328

petrjasek/superdesk-ntb,ioanpocol/superdesk-ntb,superdesk/superdesk-ntb,ioanpocol/superdesk-ntb,superdesk/superdesk-ntb,petrjasek/superdesk-ntb,ioanpocol/superdesk-ntb,superdesk/superdesk-ntb,petrjasek/superdesk-ntb,superdesk/superdesk-ntb,petrjasek/superdesk-ntb