Use golang:alpine as docker base image

This shrinks the image by about 0.5. Thanks @fbochu.

Author: Kelly Norton

Dockerfile

@@ -1,8 +1,9 @@
-FROM golang
+FROM golang:alpine
 
 COPY . /go/src/github.com/etsy/hound
 ONBUILD COPY config.json /hound/
-RUN go-wrapper install github.com/etsy/hound/cmds/houndd
+RUN apk update && apk add git subversion mercurial bzr
+RUN go install github.com/etsy/hound/cmds/houndd
 
 EXPOSE 6080