docker ps: print warning if both --format and --quiet are set

Of both "--quiet" and "--format" are set, --quiet takes precedence. This
patch adds a warning to inform the user that their custom format is not
used:

    docker ps --format='{{.Image}}'
    ubuntu:22.04
    alpine

    docker ps --format='{{.Image}}' --quiet
    WARNING: Ignoring custom format, because both --format and --quiet are set.
    40111f61d5c5
    482efdf39fac

The warning is printed on STDERR, so can be redirected:

    docker ps --format='{{.Image}}' --quiet 2> /dev/null
    40111f61d5c5
    482efdf39fac

The warning is only shown if the format is set using the "--format" option.
No warning is shown if a custom format is set through the CLI configuration
file:

    mkdir -p ~/.docker/
    echo '{"psFormat": "{{.Image}}"}' > ~/.docker/config.json

    docker ps
    ubuntu:22.04
    alpine

    docker ps --quiet
    40111f61d5c5
    482efdf39fac

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

cli/command/container/list.go

@@ -120,6 +120,8 @@ func runPs(dockerCli command.Cli, options *psOptions) error {
 	if len(options.format) == 0 {
 		// load custom psFormat from CLI config (if any)
 		options.format = dockerCli.ConfigFile().PsFormat
+	} else if options.quiet {
+		_, _ = dockerCli.Err().Write([]byte("WARNING: Ignoring custom format, because both --format and --quiet are set.\n"))
 	}
 
 	listOptions, err := buildContainerListOptions(options)

cli/command/container/list_test.go

@@ -324,6 +324,7 @@ func TestContainerListWithFormat(t *testing.T) {
 		assert.Check(t, cmd.Flags().Set("format", "{{ .Names }} {{ .Image }} {{ .Labels }}"))
 		assert.Check(t, cmd.Flags().Set("quiet", "true"))
 		assert.NilError(t, cmd.Execute())
+		assert.Equal(t, cli.ErrBuffer().String(), "WARNING: Ignoring custom format, because both --format and --quiet are set.\n")
 		golden.Assert(t, cli.OutBuffer().String(), "container-list-quiet.golden")
 	})
 }