Fix: Set value for environment variable

The docker build command was failing midway since `ENV` needs two
parameters - the name of the variable, and its value. In the
dockerfile, the variable had no value set against it - causing
the error

Fix this by introducing a new build arg, and setting this as the
value for the environment variable

Dockerfile

@@ -6,7 +6,8 @@ RUN apk add --update make
 RUN mkdir /silver-surfer
 WORKDIR /silver-surfer
 ADD . /silver-surfer/
-ENV GITHUB_TOKEN
+ARG AUTH_TOKEN
+ENV GITHUB_TOKEN=${AUTH_TOKEN}
 ARG RELEASE
 RUN if [ "$RELEASE" = "goreleaser" ]; then echo `make release`; fi
 RUN GOOS=linux make