Curl is already installed as default

Dockerfile

@@ -1,9 +1,4 @@
 FROM golang:1.9
 WORKDIR /go/src/fluux.io/xmpp
 COPY . ./
-RUN apt-get update \
-    && apt-get install -y \
-       curl curl \
-       --no-install-recommends \
-    && rm -rf /var/lib/apt/lists/*
 RUN go get -t -v ./...