Issue title: docker-standard Cannot connect to the Docker daemon at unix:///var/run/docker.sock.
 Issue body: 
I create a vsts-agent container with command:
```
  docker run \
  -e VSTS_ACCOUNT=... \
  -e VSTS_TOKEN=.... \
  -e VSTS_AGENT='$(hostname)-agent' \
  -e VSTS_POOL=docker \
  -e VSTS_WORK='/var/vsts/$VSTS_AGENT' \
  -v /var/vsts:/var/vsts \
  --restart=always \
  -d \
  -it microsoft/vsts-agent:ubuntu-16.04-docker-17.06.0-ce-standard
```
but when I build an image use this agent,I got some error:
```
2018-04-17T13:53:02.3856500Z ##[section]Starting: Build an image
2018-04-17T13:53:02.4097938Z ==============================================================================
2018-04-17T13:53:02.4110018Z Task         : Docker
2018-04-17T13:53:02.4122104Z Description  : Build, tag, push, or run Docker images, or run a Docker command. Task can be used with Docker or Azure Container registry.
2018-04-17T13:53:02.4134161Z Version      : 0.3.11
2018-04-17T13:53:02.4145990Z Author       : Microsoft Corporation
2018-04-17T13:53:02.4157924Z Help         : [More Information](https://go.microsoft.com/fwlink/?linkid=848006)
2018-04-17T13:53:02.4170025Z ==============================================================================
2018-04-17T13:53:02.8690020Z eb9326ac-2bf6-4dd2-bb4a-a9bab2fa565f exists true
2018-04-17T13:53:03.0318911Z [command]/usr/local/bin/docker build -f /var/vsts/8e5a00f65bff-agent/1/s/WebApplication2/Dockerfile -t 116.69.203.115/demo/ddemo:0.03 -t 116.69.203.115/demo/ddemo /var/vsts/8e5a00f65bff-agent/1/s/WebApplication2
2018-04-17T13:53:03.0334993Z Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
2018-04-17T13:53:03.0420223Z ##[error]Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
2018-04-17T13:53:03.0472064Z ##[error]/usr/local/bin/docker failed with return code: 1
2018-04-17T13:53:03.0709145Z ##[section]Finishing: Build an image

```

someone kown why?
 Comments: 
 Comment 0: @dakale  bingo, thanks
 Comment 1: What is the volume mapping if running on a Windows host?
{'source': 'GithubIssues', 'repo_name_with_owner': 'Microsoft/vsts-agent-docker', 'issue_id': 315081743, 'repo_stars': 174.0}