This fixes a panic when an invalid "device cgroup rule" is passed, resulting
in an "index out of range".

This bug was introduced in the original implementation in 1756af6,
but was not reproducible when using the CLI, because the same commit also added
client-side validation on the flag before making an API request. The following
example, uses an invalid rule (`c *:*  rwm` - two spaces before the permissions);

```console
$ docker run --rm --network=host --device-cgroup-rule='c *:*  rwm' busybox
invalid argument "c *:*  rwm" for "--device-cgroup-rule" flag: invalid device cgroup format 'c *:*  rwm'
```

Doing the same, but using the API results in a daemon panic when starting the container;

Create a container with an invalid device cgroup rule:

```console
curl -v \
  --unix-socket /var/run/docker.sock \
  "http://localhost/v1.41/containers/create?name=foobar" \
  -H "Content-Type: application/json" \
  -d '{"Image":"busybox:latest", "HostConfig":{"DeviceCgroupRules": ["c *:*  rwm"]}}'
```

Start the container:

```console
curl -v \
  --unix-socket /var/run/docker.sock \
  -X POST \
  "http://localhost/v1.41/containers/foobar/start"
```

Observe the daemon logs:

```
2021-01-22 12:53:03.313806 I | http: panic serving @: runtime error: index out of range [0] with length 0
goroutine 571 [running]:
net/http.(*conn).serve.func1(0xc000cb2d20)
	/usr/local/go/src/net/http/server.go:1795 +0x13b
panic(0x2f32380, 0xc000aebfc0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b6
github.com/docker/docker/oci.AppendDevicePermissionsFromCgroupRules(0xc000175c00, 0x8, 0x8, 0xc0000bd380, 0x1, 0x4, 0x0, 0x0, 0xc0000e69c0, 0x0, ...)
	/go/src/github.com/docker/docker/oci/oci.go:34 +0x64f
```

This patch:

- fixes the panic, allowing the daemon to return an error on container start
- adds a unit-test to validate various permutations
- adds a "todo" to verify the regular expression (and handling) of the "a" (all) value

We should also consider performing this validation when _creating_ the container,
so that an error is produced early.

Signed-off-by: Sebastiaan van Stijn <[email protected]>