Reinstate EXPOSE instruction

This ensures that the relevant port is published when using `docker run -P`, Traefik's Docker integration etc

packaging/docker/Dockerfile

@@ -9,6 +9,7 @@ RUN go build \
     main.go
 
 FROM alpine:latest
+EXPOSE 8080
 ENV LISTEN_ADDR 0.0.0.0:8080
 RUN apk --no-cache add ca-certificates tzdata
 COPY --from=build /go/src/app/miniflux /usr/bin/miniflux