cli/command/completion: add EnvVarNames utility

EnvVarNames offers completion for environment-variable names. This
completion can be used for "--env" and "--build-arg" flags, which
allow obtaining the value of the given environment-variable if present
in the local environment, so we only should complete the names of the
environment variables, and not their value. This also prevents the
completion script from printing values of environment variables
containing sensitive values.

For example;

    export MY_VAR=hello
    docker run --rm --env MY_VAR alpine printenv MY_VAR
    hello

Before this patch:

    docker run --env GO
    GO111MODULE=auto        GOLANG_VERSION=1.21.12  GOPATH=/go              GOTOOLCHAIN=local

With this patch:

    docker run --env GO<tab>
    GO111MODULE     GOLANG_VERSION  GOPATH          GOTOOLCHAIN

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

cli/command/completion/functions.go

@@ -2,6 +2,7 @@ package completion
 
 import (
 	"os"
+	"strings"
 
 	"github.com/docker/cli/cli/command/formatter"
 	"github.com/docker/docker/api/types/container"
@@ -105,6 +106,29 @@ func NetworkNames(dockerCLI APIClientProvider) ValidArgsFn {
 	}
 }
 
+// EnvVarNames offers completion for environment-variable names. This
+// completion can be used for "--env" and "--build-arg" flags, which
+// allow obtaining the value of the given environment-variable if present
+// in the local environment, so we only should complete the names of the
+// environment variables, and not their value. This also prevents the
+// completion script from printing values of environment variables
+// containing sensitive values.
+//
+// For example;
+//
+//	export MY_VAR=hello
+//	docker run --rm --env MY_VAR alpine printenv MY_VAR
+//	hello
+func EnvVarNames(_ *cobra.Command, _ []string, _ string) (names []string, _ cobra.ShellCompDirective) {
+	envs := os.Environ()
+	names = make([]string, 0, len(envs))
+	for _, env := range envs {
+		name, _, _ := strings.Cut(env, "=")
+		names = append(names, name)
+	}
+	return names, cobra.ShellCompDirectiveNoFileComp
+}
+
 // FileNames is a convenience function to use [cobra.ShellCompDirectiveDefault],
 // which indicates to let the shell perform its default behavior after
 // completions have been provided.

cli/command/container/exec.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"os"
 
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
@@ -78,9 +77,7 @@ func NewExecCommand(dockerCli command.Cli) *cobra.Command {
 	flags.StringVarP(&options.Workdir, "workdir", "w", "", "Working directory inside the container")
 	flags.SetAnnotation("workdir", "version", []string{"1.35"})
 
-	_ = cmd.RegisterFlagCompletionFunc("env", func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
-		return os.Environ(), cobra.ShellCompDirectiveNoFileComp
-	})
+	_ = cmd.RegisterFlagCompletionFunc("env", completion.EnvVarNames)
 	_ = cmd.RegisterFlagCompletionFunc("env-file", completion.FileNames)
 
 	return cmd

cli/command/container/run.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"os"
 	"strings"
 	"syscall"
 
@@ -70,9 +69,7 @@ func NewRunCommand(dockerCli command.Cli) *cobra.Command {
 	command.AddTrustVerificationFlags(flags, &options.untrusted, dockerCli.ContentTrustEnabled())
 	copts = addFlags(flags)
 
-	_ = cmd.RegisterFlagCompletionFunc("env", func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
-		return os.Environ(), cobra.ShellCompDirectiveNoFileComp
-	})
+	_ = cmd.RegisterFlagCompletionFunc("env", completion.EnvVarNames)
 	_ = cmd.RegisterFlagCompletionFunc("env-file", completion.FileNames)
 	_ = cmd.RegisterFlagCompletionFunc("network", completion.NetworkNames(dockerCli))
 	return cmd