Update: Ensure docker build fails without AUTH_TOKEN build arg

Since the `GITHUB_TOKEN` being used as the environment variable in
the docker image was needed (and not optional) - allowing docker
builds to succeed, only to result in a crash when the container was
run would be a waste of time.

With this commit, the `AUTH_TOKEN` argument is mandatory for the
`docker build` command to succeed. This greatly cuts down the
response time to detect an error with the build command

Dockerfile

@@ -7,6 +7,7 @@ RUN mkdir /silver-surfer
 WORKDIR /silver-surfer
 ADD . /silver-surfer/
 ARG AUTH_TOKEN
+RUN test -n "$AUTH_TOKEN"
 ENV GITHUB_TOKEN=${AUTH_TOKEN}
 ARG RELEASE
 RUN if [ "$RELEASE" = "goreleaser" ]; then echo `make release`; fi