Add env var that describes QA release to Dockerfile

qa/Dockerfile

 FROM ruby:2.3
 LABEL maintainer "Grzegorz Bizon <grzegorz@gitlab.com>"
 
+ENV GITLAB_RELEASE CE
+
 RUN sed -i "s/httpredir.debian.org/ftp.us.debian.org/" /etc/apt/sources.list && \
     apt-get update && apt-get install -y --force-yes \
       libqt5webkit5-dev qt5-qmake qt5-default build-essential xvfb git && \