Add Logspout extension (#133)

extensions/README.md

+Third-party extensions that enable extra integrations with the ELK stack.

extensions/logspout/Dockerfile

+# uses ONBUILD instructions described here:
+# https://github.com/gliderlabs/logspout/tree/master/custom
+
+FROM gliderlabs/logspout:master
+ENV SYSLOG_FORMAT rfc3164

extensions/logspout/README.md

+# Logspout extension
+
+Logspout collects all Docker logs using the Docker logs API, and forwards them to Logstash without any additional
+configuration.
+
+## Usage
+
+If you want to include the Logspout extension, ensure the additional `logspout-compose.yml` file is included in the
+command line parameters:
+
+```bash
+$ docker-compose -f docker-compose.yml -f logspout-compose.yml up
+```
+
+In your Logstash pipeline configuration, enable the `udp` input and set the input codec to `json`:
+
+```
+input {
+  udp {
+    port  => 5000
+    codec => json
+  }
+}
+```
+
+## Documentation
+
+https://github.com/looplab/logspout-logstash

extensions/logspout/build.sh

+#!/bin/sh
+
+# unmodified from:
+# https://github.com/gliderlabs/logspout/blob/67ee3831cbd0594361bb3381380c65bdbeb3c20f/custom/build.sh
+
+set -e
+apk add --update go git mercurial build-base
+mkdir -p /go/src/github.com/gliderlabs
+cp -r /src /go/src/github.com/gliderlabs/logspout
+cd /go/src/github.com/gliderlabs/logspout
+export GOPATH=/go
+go get
+go build -ldflags "-X main.Version=$1" -o /bin/logspout
+apk del go git mercurial build-base
+rm -rf /go /var/cache/apk/* /root/.glide
+
+# backwards compatibility
+ln -fs /tmp/docker.sock /var/run/docker.sock

extensions/logspout/logspout-compose.yml

+version: '2'
+
+services:
+  logspout:
+    build:
+      context: extensions/logspout
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+    environment:
+      ROUTE_URIS: logstash://logstash:5000
+      LOGSTASH_TAGS: docker-elk
+    networks:
+      - elk
+    depends_on:
+      - logstash
+    restart: on-failure

extensions/logspout/modules.go

+package main
+
+// installs the Logstash adapter for Logspout, and required dependencies
+// https://github.com/looplab/logspout-logstash
+import (
+	_ "github.com/looplab/logspout-logstash"
+	_ "github.com/gliderlabs/logspout/transports/udp"
+	_ "github.com/gliderlabs/logspout/transports/tcp"
+)