Adding REF to GitHub repo HEAD for cache busting

After installing requirements, check if  HEAD has changed and bust cache for RUN instruction for wget  archive

Dockerfile

@@ -36,6 +36,9 @@ ARG REQUIREMENTS_URL=https://raw.githubusercontent.com/digitalocean/netbox/$BRAN
 ADD ${REQUIREMENTS_URL} requirements.txt
 RUN pip install -r requirements.txt
 
+ARG REF_URL=https://api.github.com/repos/digitalocean/netbox/git/refs/heads/$BRANCH
+ADD ${REF_URL} version.json
+
 ARG URL=https://github.com/digitalocean/netbox/archive/$BRANCH.tar.gz
 RUN wget -q -O - "${URL}" | tar xz \
   && mv netbox* netbox