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/completion.go

@@ -3,6 +3,7 @@ package container
 import (
 	"github.com/docker/cli/cli/command/completion"
 	"github.com/docker/docker/api/types/container"
+	"github.com/moby/sys/signal"
 	"github.com/spf13/cobra"
 )
 
@@ -82,3 +83,12 @@ func completeLinuxCapabilityNames(cmd *cobra.Command, args []string, toComplete
 func completeRestartPolicies(cmd *cobra.Command, args []string, toComplete string) (names []string, _ cobra.ShellCompDirective) {
 	return completion.FromList(restartPolicies...)(cmd, args, toComplete)
 }
+
+func completeSignals(cmd *cobra.Command, args []string, toComplete string) (names []string, _ cobra.ShellCompDirective) {
+	// TODO(thaJeztah): do we want to provide the full list here, or a subset?
+	signalNames := make([]string, 0, len(signal.SignalMap))
+	for k := range signal.SignalMap {
+		signalNames = append(signalNames, k)
+	}
+	return completion.FromList(signalNames...)(cmd, args, toComplete)
+}