Allow easier LISTEN_ADDRESS overrides to dockerfile

Because listen.address was hard coded in the Docker file, it would override any ENVVAR set in any
other docker file, this made it hard to change the default listening port to anything else. By switching
to `ENV LISTEN_ADDRESS=0.0.0.0:9094` people should be able to override the address that
the docker file listens on much easier.

This closes benjojo#12 
This closes benjojo#11

Author: benjojo

Dockerfile

@@ -22,7 +22,8 @@ COPY --from=builder /etc/passwd /etc/passwd
 # Copy our static executable
 COPY --from=builder /go/bin/alertmanager-discord /go/bin/alertmanager-discord
 
+ENV LISTEN_ADDRESS=0.0.0.0:9094
 EXPOSE 9094
 USER appuser
-ENTRYPOINT ["/go/bin/alertmanager-discord", "-listen.address", "0.0.0.0:9094"]
+ENTRYPOINT ["/go/bin/alertmanager-discord"]