docker version: add "context" to output

This adds the currently selected "docker context" to the output
of "docker version", which allows users to see which context
is selected to produce the version output, and can be used (for
example), to set the prompt to the currently selected context:

(in `~/.bashrc`):

```bash
function docker_context_prompt() {
        PS1="context: $(docker version --format='{{.Client.Context}}')> "
}

PROMPT_COMMAND=docker_context_prompt
```

After reloading the `~/.bashrc`, the prompt now shows the currently selected
`docker context`:

```bash
$ source ~/.bashrc
context: default> docker context create --docker host=unix:///var/run/docker.sock my-context
my-context
Successfully created context "my-context"
context: default> docker context use my-context
my-context
Current context is now "my-context"
context: my-context> docker context use default
default
Current context is now "default"
context: default>
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/testdata/docker-client-version.golden

@@ -5,6 +5,7 @@ Client:
  Git commit:        deadbeef
  Built:             Wed May 30 22:21:05 2018
  OS/Arch:           linux/amd64
+ Context:           my-context
  Experimental:      true
 
 Server: Docker Enterprise Edition (EE) 2.0

cli/command/system/version.go

@@ -32,6 +32,7 @@ Client:{{if ne .Platform.Name ""}} {{.Platform.Name}}{{end}}
  Git commit:	{{.GitCommit}}
  Built:	{{.BuildTime}}
  OS/Arch:	{{.Os}}/{{.Arch}}
+ Context:	{{.Context}}
  Experimental:	{{.Experimental}}
 {{- end}}
 
@@ -80,6 +81,7 @@ type clientVersion struct {
 	Os                string
 	Arch              string
 	BuildTime         string `json:",omitempty"`
+	Context           string
 	Experimental      bool
 }
 
@@ -147,6 +149,7 @@ func runVersion(dockerCli command.Cli, opts *versionOptions) error {
 			Os:                runtime.GOOS,
 			Arch:              runtime.GOARCH,
 			Experimental:      dockerCli.ClientInfo().HasExperimental,
+			Context:           dockerCli.CurrentContext(),
 		},
 	}
 

cli/command/system/version_test.go

@@ -41,6 +41,7 @@ func TestVersionAlign(t *testing.T) {
 			Os:                "linux",
 			Arch:              "amd64",
 			BuildTime:         "Wed May 30 22:21:05 2018",
+			Context:           "my-context",
 			Experimental:      true,
 		},
 		Server: &types.Version{},