Pointing Dockerfile to 'v2-beta' branch

Was pointing to a particular tag instead.

Dockerfile

@@ -2,7 +2,7 @@ FROM python:2.7-wheezy
 
 WORKDIR /opt/netbox
 
-ARG BRANCH=v2.0-beta1
+ARG BRANCH=v2-beta
 ARG URL=https://github.com/digitalocean/netbox.git
 RUN git clone --depth 1 $URL -b $BRANCH .  && \
     apt-get update -qq && apt-get install -y libldap2-dev libsasl2-dev libssl-dev graphviz && \