Add checks to verifiy if a new build is needed

This checks if the source materials (python image, Netbox commit,
netbox-docker commit) have changed since the last build. This check is done
by comparing the digest and commit ids from the previous image with the
given tag to the current values taken from the Git and Docker repositories.

The checks are only performed for builds by the automated builds on Github.

Dockerfile

@@ -1,4 +1,4 @@
-ARG FROM=python:3.7-alpine
+ARG FROM
 FROM ${FROM} as builder
 
 RUN apk add --no-cache \