Artifacts are not executable

### Nomad version
Nomad v0.5.6

### Operating system and Environment details
* Ubuntu 16.04.2 LTS (Xenial Xerus)
* Docker 17.05.0-ce

### Issue
Downloaded artifacts are not executable in a docker container. It works with raw_fork but not with the Docker driver.
I get the following message when using Ubuntu with Vagrant. (I also tested it with Docker for Mac):

```
f0b494360b302168d8334382dbba185c66b77143729a4c1b6b223d10e44a26cd: API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
```

### Reproduction steps

My go program I'm trying to run as "app":
```go
package main

import "log"

func main() {
log.Println("DONE! :)")
}
```

I build this program with the following command:
```
GOOS=linux GOARCH=amd64 go build -o app app.go
```

Then I serve the binary via a simple HTTP sever and run the job file (below).

### Nomad Server logs (if appropriate)
```
2017/05/26 19:21:59.951926 [DEBUG] http: Request /v1/evaluation/2cfa116e-4c0b-3b14-6c22-84acf6a5137f/allocations (157.131µs)
2017/05/26 19:22:00.999816 [DEBUG] driver.docker: docker pull alpine:latest succeeded
2017/05/26 19:22:01.001897 [DEBUG] driver.docker: image "alpine" (sha256:a41a7446062d197dd4b21b38122dcc7b2399deb0750c4110925a7dd37c80f118) reference count incremented: 1
2017/05/26 19:22:01 [DEBUG] plugin: starting plugin: /home/ubuntu/nomad []string{"/home/ubuntu/nomad", "executor", "{\"LogFile\":\"/tmp/NomadClient799156968/1ae5668e-74c0-050d-627d-fd236faeecd1/app/executor.out\",\"LogLevel\":\"DEBUG\"}"}
2017/05/26 19:22:01 [DEBUG] plugin: waiting for RPC address for: /home/ubuntu/nomad
2017/05/26 19:22:01 [DEBUG] plugin: nomad: 2017/05/26 19:22:01 [DEBUG] plugin: plugin address: unix /tmp/plugin476509569
2017/05/26 19:22:01.043571 [DEBUG] driver.docker: Setting default logging options to syslog and unix:///tmp/plugin112291564
2017/05/26 19:22:01.043616 [DEBUG] driver.docker: Using config for logging: {Type:syslog ConfigRaw:[] Config:map[syslog-address:unix:///tmp/plugin112291564]}
2017/05/26 19:22:01.043622 [DEBUG] driver.docker: using 268435456 bytes memory for app
2017/05/26 19:22:01.043626 [DEBUG] driver.docker: using 500 cpu shares for app
2017/05/26 19:22:01.043664 [DEBUG] driver.docker: binding directories []string{"/tmp/NomadClient799156968/1ae5668e-74c0-050d-627d-fd236faeecd1/alloc:/alloc", "/tmp/NomadClient799156968/1ae5668e-74c0-050d-627d-fd236faeecd1/app/local:/local", "/tmp/NomadClient799156968/1ae5668e-74c0-050d-627d-fd236faeecd1/app/secrets:/secrets"} for app
2017/05/26 19:22:01.043676 [DEBUG] driver.docker: networking mode not specified; defaulting to bridge
2017/05/26 19:22:01.043684 [DEBUG] driver.docker: allocated port 192.168.178.29:40699 -> 40699 (mapped)
2017/05/26 19:22:01.043690 [DEBUG] driver.docker: exposed port 40699
2017/05/26 19:22:01.043744 [DEBUG] driver.docker: setting container startup command to: ./local/app
2017/05/26 19:22:01.043760 [DEBUG] driver.docker: setting container name to: app-1ae5668e-74c0-050d-627d-fd236faeecd1
2017/05/26 19:22:01.084600 [INFO] driver.docker: created container f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112
2017/05/26 19:22:01.507951 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 1): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:02.939463 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 2): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:04.331313 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 3): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:05.746869 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 4): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:07.171912 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 5): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:08.579853 [DEBUG] driver.docker: failed to start container "f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112" (attempt 6): API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:08.579976 [ERR] driver.docker: failed to start container f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112: API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:08 [DEBUG] plugin: /home/ubuntu/nomad: plugin process exited
2017/05/26 19:22:08.582071 [WARN] client: failed to start task "app" for alloc "1ae5668e-74c0-050d-627d-fd236faeecd1": Failed to start container f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112: API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
2017/05/26 19:22:08.582646 [INFO] client: Not restarting task: app for alloc: 1ae5668e-74c0-050d-627d-fd236faeecd1
2017/05/26 19:22:08.582877 [DEBUG] driver.docker: image id "sha256:a41a7446062d197dd4b21b38122dcc7b2399deb0750c4110925a7dd37c80f118" reference count decremented: 0
2017/05/26 19:22:08.583423 [INFO] client: marking allocation 1ae5668e-74c0-050d-627d-fd236faeecd1 for GC
2017/05/26 19:22:08.584152 [INFO] client: marking allocation 1ae5668e-74c0-050d-627d-fd236faeecd1 for GC
2017/05/26 19:22:08.746745 [DEBUG] client: updated allocations at index 12 (total 1) (pulled 0) (filtered 1)
2017/05/26 19:22:08.746897 [DEBUG] client: allocs: (added 0) (removed 0) (updated 0) (ignore 1)
```

### Nomad Client logs (if appropriate)
```
➜ Desktop ./nomad alloc-status --address=http://192.168.178.29:4646 1ae5668e
ID = 1ae5668e
Eval ID = 2cfa116e
Name = app.app[0]
Node ID = e6e35f7b
Job ID = app
Client Status = failed
Client Description =
Desired Status = run
Desired Description =
Created At = 05/26/17 21:21:58 CEST

Task "app" is "dead"
Task Resources
CPU Memory Disk IOPS Addresses
500 MHz 256 MiB 300 MiB 0 db: 192.168.178.29:40699

Recent Events:
Time Type Description
05/26/17 21:22:08 CEST Not Restarting Policy allows no restarts
05/26/17 21:22:08 CEST Driver Failure failed to start task "app" for alloc "1ae5668e-74c0-050d-627d-fd236faeecd1": Failed to start container f909e9e0fc21cf502b542d24e1ce3733b97ff4dfcddd2805195f7605679e5112: API error (500): {"message":"oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"./local/app\\\": permission denied\"\n"}
05/26/17 21:21:58 CEST Driver Downloading image alpine:latest
05/26/17 21:21:58 CEST Downloading Artifacts Client is downloading artifacts
05/26/17 21:21:58 CEST Task Setup Building Task Directory
05/26/17 21:21:58 CEST Received Task received by client
```

### Job file (if appropriate)
```
job "app" {
datacenters = ["dc1"]

type = "batch"

update {
stagger = "10s"
max_parallel = 1
}

group "app" {
count = 1

restart {
attempts = 0
interval = "5m"
delay = "25s"
mode = "fail"
}

task "app" {
driver = "docker"

config {
image = "alpine"
command = "./local/app"
}

artifact {
source = "http://localhost:8333/app"
}

logs {
max_files = 1
max_file_size = 15
}

resources {
cpu = 500 # 500 MHz
memory = 256 # 256MB
network {
mbits = 10
port "db" {}
}
}
}
}
}
```

Start by reproducing the reported job with Nomad v0.5.6, the Docker driver, the Alpine image, and the HTTP artifact source. Trace artifact download and Docker task startup handling; done means the downloaded Go binary can be launched through command = "./local/app" without a permission-denied error.

Written by the indexing model from the issue text.