Data race in add_docker_metadata test

[andrewkroh]

The failure was observed while testing the 6.0 branch on Jenkins. This needs to be investigated further.

https://beats-ci.elastic.co/job/elastic+beats+pull-request+multijob-linux/1465/beat=libbeat,label=ubuntu/console

command [go test -race -cover -coverprofile /tmp/gotestcover-055221072 github.com/elastic/beats/libbeat/processors/add_docker_metadata]: exit status 1
==================
WARNING: DATA RACE
Write at 0x00c42034c2d0 by goroutine 44:
  runtime.mapassign()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/runtime/hashmap.go:485 +0x0
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.(*watcher).watch()
      github.com/elastic/beats/libbeat/processors/add_docker_metadata/_test/_obj_test/watcher.go:222 +0xaf2

Previous read at 0x00c42034c2d0 by goroutine 43:
  runtime.mapaccess2_faststr()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/runtime/hashmap_fast.go:317 +0x0
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.(*watcher).Container()
      github.com/elastic/beats/libbeat/processors/add_docker_metadata/_test/_obj_test/watcher.go:114 +0x13d
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.TestWatcherKill()
      /var/lib/jenkins/workspace/elastic+beats+pull-request+multijob-linux/beat/libbeat/label/ubuntu/src/github.com/elastic/beats/libbeat/processors/add_docker_metadata/watcher_test.go:178 +0x3d2
  testing.tRunner()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:657 +0x107

Goroutine 44 (running) created at:
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.(*watcher).Start()
      github.com/elastic/beats/libbeat/processors/add_docker_metadata/_test/_obj_test/watcher.go:165 +0x634
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.runWatcher()
      /var/lib/jenkins/workspace/elastic+beats+pull-request+multijob-linux/beat/libbeat/label/ubuntu/src/github.com/elastic/beats/libbeat/processors/add_docker_metadata/watcher_test.go:200 +0x20e
  github.com/elastic/beats/libbeat/processors/add_docker_metadata.TestWatcherKill()
      /var/lib/jenkins/workspace/elastic+beats+pull-request+multijob-linux/beat/libbeat/label/ubuntu/src/github.com/elastic/beats/libbeat/processors/add_docker_metadata/watcher_test.go:174 +0x396
  testing.tRunner()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:657 +0x107

Goroutine 43 (running) created at:
  testing.(*T).Run()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:697 +0x543
  testing.runTests.func1()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:882 +0xaa
  testing.tRunner()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:657 +0x107
  testing.runTests()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:888 +0x4e0
  testing.(*M).Run()
      /var/lib/jenkins/.gvm/versions/go1.8.3.linux.amd64/src/testing/testing.go:822 +0x1c3
  main.main()
      github.com/elastic/beats/libbeat/processors/add_docker_metadata/_test/_testmain.go:110 +0x33d
==================
--- FAIL: TestWatcherKill (1.05s)
	testing.go:610: race detected during execution of test
FAIL