Executor virtualbox doesn not work when the runner is a docker container

When you start and register a new runner in a docker container (using the gitlab/gitlab-runner), the virtualbox executor does not work.

The vboxmanage is not distributed with the runner, so it can't find it:

Checking for builds... received                     runner=7048a572
gitlab-ci-multi-runner 1.2.0~beta.41.g2a5fc0f (2a5fc0f)  build=167 runner=7048a572
167 Submitting build to coordinator... ok           runner=7048a572
panic: exec: "vboxmanage": executable file not found in $PATH

goroutine 10 [running]:
panic(0xa37a40, 0xc82050e2a0)
	/usr/local/go/src/runtime/panic.go:464 +0x3e6
gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/virtualbox.(*executor).Prepare(0xc82009c500, 0xc820012ff0, 0xc8202dc000, 0xc820167200, 0x0, 0x0)
	/go/src/gitlab.com/gitlab-org/gitlab-ci-multi-runner/executors/virtualbox/executor_virtualbox.go:137 +0x4ea
gitlab.com/gitlab-org/gitlab-ci-multi-runner/common.(*Build).Run(0xc820167200, 0xc820012ff0, 0x7f6ef79d3c88, 0xc8201fa1e0, 0x0, 0x0)
	/go/src/gitlab.com/gitlab-org/gitlab-ci-multi-runner/common/build.go:115 +0x44f
gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands.(*RunCommand).processRunner(0xc8200e9a00, 0x0, 0xc8202dc000, 0x0, 0x0)
	/go/src/gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands/multi.go:132 +0x4ee
gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands.(*RunCommand).processRunners(0xc8200e9a00, 0x0, 0xc82000ed80, 0xc82000ecc0)
	/go/src/gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands/multi.go:140 +0x22c
created by gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands.(*RunCommand).startWorkers
	/go/src/gitlab.com/gitlab-org/gitlab-ci-multi-runner/commands/multi.go:156 +0x96

And what's worse: the runner returns 0:

Status	Build ID	Commit	Ref	Stage	Name	Duration	Finished at
 success	#167	de641e24	1.0.3	test	vbtest virtualbox	1 second	about a minute ago