move docker_tls_args into weaveutil

and also give users a clue what went wrong when docker isn't running

prog/weaveexec/Dockerfile

@@ -20,7 +20,7 @@ RUN apk add --update \
   && rm -rf /var/cache/apk/*
 
 ADD ./weave ./sigproxy ./weaveproxy ./symlink /home/weave/
-ADD ./weaveutil ./docker_tls_args /usr/bin/
+ADD ./weaveutil /usr/bin/
 ADD ./weavewait /w/w
 ADD ./weavewait_noop /w-noop/w
 ADD ./weavewait_nomcast /w-nomcast/w

prog/weaveutil/docker_tls_args.go

@@ -13,14 +13,18 @@ import (
 	"strings"
 )
 
-func main() {
+func dockerTLSArgs(args []string) error {
+	if len(args) > 0 {
+		cmdUsage("docker-tls-args", "")
+	}
 	procRoot := os.Getenv("PROCFS")
 	if procRoot == "" {
 		procRoot = "/proc"
 	}
-
 	dirEntries, err := ioutil.ReadDir(procRoot)
-	checkErr(err)
+	if err != nil {
+		return err
+	}
 
 	for _, dirEntry := range dirEntries {
 		dirName := dirEntry.Name()
@@ -63,15 +67,8 @@ func main() {
 		}
 
 		fmt.Println(strings.Join(tlsArgs, " "))
-		os.Exit(0)
+		return nil
 	}
 
-	os.Exit(1)
-}
-
-func checkErr(err error) {
-	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
-	}
+	return fmt.Errorf("cannot locate running docker daemon")
 }

prog/weaveutil/main.go

@@ -10,8 +10,9 @@ var commands map[string]func([]string) error
 
 func init() {
 	commands = map[string]func([]string) error{
-		"help":     help,
-		"netcheck": netcheck,
+		"help":            help,
+		"netcheck":        netcheck,
+		"docker-tls-args": dockerTLSArgs,
 	}
 }