Add `--file` flag for `docker secret create` command

This fix tries to address the issue raised in 28581 and 28927
where it is not possible to create a secret from a file (only
through STDIN).

This fix add a flag `--file` to `docker secret create` so that
it is possible to create a secret from a file with:
```
docker secret create --file secret.in secret.name
```

or
```
echo TEST | docker secret create --file - secret.name
```

Related docs has been updated.

An integration test has been added to cover the changes.

This fix fixes 28581.
This fix is related to 28927.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

command/secret/create.go

@@ -2,13 +2,14 @@ package secret
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
-	"os"
 
 	"github.com/docker/docker/api/types/swarm"
 	"github.com/docker/docker/cli"
 	"github.com/docker/docker/cli/command"
 	"github.com/docker/docker/opts"
+	"github.com/docker/docker/pkg/system"
 	runconfigopts "github.com/docker/docker/runconfig/opts"
 	"github.com/spf13/cobra"
 	"golang.org/x/net/context"
@@ -16,6 +17,7 @@ import (
 
 type createOptions struct {
 	name   string
+	file   string
 	labels opts.ListOpts
 }
 
@@ -26,7 +28,7 @@ func newSecretCreateCommand(dockerCli *command.DockerCli) *cobra.Command {
 
 	cmd := &cobra.Command{
 		Use:   "create [OPTIONS] SECRET",
-		Short: "Create a secret using stdin as content",
+		Short: "Create a secret from a file or STDIN as content",
 		Args:  cli.ExactArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			createOpts.name = args[0]
@@ -35,6 +37,7 @@ func newSecretCreateCommand(dockerCli *command.DockerCli) *cobra.Command {
 	}
 	flags := cmd.Flags()
 	flags.VarP(&createOpts.labels, "label", "l", "Secret labels")
+	flags.StringVarP(&createOpts.file, "file", "f", "", "Read from a file or STDIN ('-')")
 
 	return cmd
 }
@@ -43,9 +46,23 @@ func runSecretCreate(dockerCli *command.DockerCli, options createOptions) error
 	client := dockerCli.Client()
 	ctx := context.Background()
 
-	secretData, err := ioutil.ReadAll(os.Stdin)
+	if options.file == "" {
+		return fmt.Errorf("Please specify either a file name or STDIN ('-') with --file")
+	}
+
+	var in io.Reader = dockerCli.In()
+	if options.file != "-" {
+		file, err := system.OpenSequential(options.file)
+		if err != nil {
+			return err
+		}
+		in = file
+		defer file.Close()
+	}
+
+	secretData, err := ioutil.ReadAll(in)
 	if err != nil {
-		return fmt.Errorf("Error reading content from STDIN: %v", err)
+		return fmt.Errorf("Error reading content from %q: %v", options.file, err)
 	}
 
 	spec := swarm.SecretSpec{