Exec: Add ability to set environment variables

Keeping the current behavior for exec, i.e., inheriting
variables from main process. New variables will be added
to current ones. If there's already a variable with that
name it will be overwritten.

Example of usage: docker exec -it -e TERM=vt100 <container> top

Closes #24355.

Signed-off-by: Jonh Wendell <jonh.wendell@redhat.com>

command/container/exec.go

@@ -11,7 +11,9 @@ import (
 	"github.com/docker/docker/cli"
 	"github.com/docker/docker/cli/command"
 	apiclient "github.com/docker/docker/client"
+	options "github.com/docker/docker/opts"
 	"github.com/docker/docker/pkg/promise"
+	runconfigopts "github.com/docker/docker/runconfig/opts"
 	"github.com/spf13/cobra"
 )
 
@@ -22,11 +24,19 @@ type execOptions struct {
 	detach      bool
 	user        string
 	privileged  bool
+	env         *options.ListOpts
+}
+
+func newExecOptions() *execOptions {
+	var values []string
+	return &execOptions{
+		env: options.NewListOptsRef(&values, runconfigopts.ValidateEnv),
+	}
 }
 
 // NewExecCommand creats a new cobra.Command for `docker exec`
 func NewExecCommand(dockerCli *command.DockerCli) *cobra.Command {
-	var opts execOptions
+	opts := newExecOptions()
 
 	cmd := &cobra.Command{
 		Use:   "exec [OPTIONS] CONTAINER COMMAND [ARG...]",
@@ -35,7 +45,7 @@ func NewExecCommand(dockerCli *command.DockerCli) *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			container := args[0]
 			execCmd := args[1:]
-			return runExec(dockerCli, &opts, container, execCmd)
+			return runExec(dockerCli, opts, container, execCmd)
 		},
 	}
 
@@ -48,6 +58,7 @@ func NewExecCommand(dockerCli *command.DockerCli) *cobra.Command {
 	flags.BoolVarP(&opts.detach, "detach", "d", false, "Detached mode: run command in the background")
 	flags.StringVarP(&opts.user, "user", "u", "", "Username or UID (format: <name|uid>[:<group|gid>])")
 	flags.BoolVarP(&opts.privileged, "privileged", "", false, "Give extended privileges to the command")
+	flags.VarP(opts.env, "env", "e", "Set environment variables")
 
 	return cmd
 }
@@ -188,5 +199,9 @@ func parseExec(opts *execOptions, container string, execCmd []string) (*types.Ex
 		}
 	}
 
+	if opts.env != nil {
+		execConfig.Env = opts.env.GetAll()
+	}
+
 	return execConfig, nil
 }