dockerfile: syntax updates and s/xz/xz-utils

This fixes the automated build on Quay.io.

Dockerfile

@@ -1,16 +1,20 @@
 FROM golang:1.5
 MAINTAINER Quentin Machu <quentin.machu@coreos.com>
 
-RUN apt-get update && apt-get install -y bzr rpm xz && apt-get autoremove -y && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+RUN apt-get update && \
+    apt-get install -y bzr rpm xz-utils && \
+    apt-get autoremove -y && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
-RUN mkdir         /db
-VOLUME            /db
-VOLUME            /config
+RUN mkdir /db
+VOLUME /db
+VOLUME /config
 
 EXPOSE 6060 6061
 
-ADD .         /go/src/github.com/coreos/clair/
-WORKDIR       /go/src/github.com/coreos/clair/
+ADD .   /go/src/github.com/coreos/clair/
+WORKDIR /go/src/github.com/coreos/clair/
 
 ENV GO15VENDOREXPERIMENT 1
 RUN go install -v github.com/coreos/clair/cmd/clair