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{},

docs/reference/commandline/version.md

@@ -39,6 +39,7 @@ Client:
  Git commit:        afacb8b
  Built:             Wed Mar 11 01:21:11 2020
  OS/Arch:           darwin/amd64
+ Context:           default
  Experimental:      true
 
 Server:
@@ -76,3 +77,47 @@ $ docker version --format '{{json .}}'
 
 {"Client":{"Platform":{"Name":"Docker Engine - Community"},"Version":"19.03.8","ApiVersion":"1.40","DefaultAPIVersion":"1.40","GitCommit":"afacb8b","GoVersion":"go1.12.17","Os":"darwin","Arch":"amd64","BuildTime":"Wed Mar 11 01:21:11 2020","Experimental":true},"Server":{"Platform":{"Name":"Docker Engine - Community"},"Components":[{"Name":"Engine","Version":"19.03.8","Details":{"ApiVersion":"1.40","Arch":"amd64","BuildTime":"Wed Mar 11 01:29:16 2020","Experimental":"true","GitCommit":"afacb8b","GoVersion":"go1.12.17","KernelVersion":"4.19.76-linuxkit","MinAPIVersion":"1.12","Os":"linux"}},{"Name":"containerd","Version":"v1.2.13","Details":{"GitCommit":"7ad184331fa3e55e52b890ea95e65ba581ae3429"}},{"Name":"runc","Version":"1.0.0-rc10","Details":{"GitCommit":"dc9208a3303feef5b3839f4323d9beb36df0a9dd"}},{"Name":"docker-init","Version":"0.18.0","Details":{"GitCommit":"fec3683"}}],"Version":"19.03.8","ApiVersion":"1.40","MinAPIVersion":"1.12","GitCommit":"afacb8b","GoVersion":"go1.12.17","Os":"linux","Arch":"amd64","KernelVersion":"4.19.76-linuxkit","Experimental":true,"BuildTime":"2020-03-11T01:29:16.000000000+00:00"}}
 ```
+
+### Print the current context
+
+The following example prints the currently used [`docker context`](context.md):
+
+```bash
+$ docker version --format='{{.Client.Context}}'
+default
+```
+
+As an example, this output can be used to dynamically change your shell prompt
+to indicate your active context. The example below illustrates how this output
+could be used when using Bash as your shell.
+
+Declare a function to obtain the current context in your `~/.bashrc`, and set
+this command as your `PROMPT_COMMAND`
+
+```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>
+```
+
+Refer to the [`docker context` section](context.md) in the command line reference
+for more information about `docker context`.