cli/command/container: add completion for --stop-signal

With this patch:

    docker run --stop-signal <TAB>
    ABRT  IOT      RTMAX-4   RTMIN     RTMIN+11  TSTP
    ALRM  KILL     RTMAX-5   RTMIN+1   RTMIN+12  TTIN
    BUS   PIPE     RTMAX-6   RTMIN+2   RTMIN+13  TTOU
    CHLD  POLL     RTMAX-7   RTMIN+3   RTMIN+14  URG
    CLD   PROF     RTMAX-8   RTMIN+4   RTMIN+15  USR1
    CONT  PWR      RTMAX-9   RTMIN+5   SEGV      USR2
    FPE   QUIT     RTMAX-10  RTMIN+6   STKFLT    VTALRM
    HUP   RTMAX    RTMAX-11  RTMIN+7   STOP      WINCH
    ILL   RTMAX-1  RTMAX-12  RTMIN+8   SYS       XCPU
    INT   RTMAX-2  RTMAX-13  RTMIN+9   TERM      XFSZ
    IO    RTMAX-3  RTMAX-14  RTMIN+10  TRAP

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit b1c0ddca02)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/kill.go

@@ -38,6 +38,9 @@ func NewKillCommand(dockerCli command.Cli) *cobra.Command {
 
 	flags := cmd.Flags()
 	flags.StringVarP(&opts.signal, "signal", "s", "", "Signal to send to the container")
+
+	_ = cmd.RegisterFlagCompletionFunc("signal", completeSignals)
+
 	return cmd
 }
 
@@ -50,7 +53,7 @@ func runKill(ctx context.Context, dockerCli command.Cli, opts *killOptions) erro
 		if err := <-errChan; err != nil {
 			errs = append(errs, err.Error())
 		} else {
-			fmt.Fprintln(dockerCli.Out(), name)
+			_, _ = fmt.Fprintln(dockerCli.Out(), name)
 		}
 	}
 	if len(errs) > 0 {