Add proper usage of usage()

icinga-build-docker

 #!/bin/bash
 
+
 set -e
 
 ## config
@@ -14,13 +15,24 @@ SCRIPT_HOME="$(dirname "$(readlink -f "$0")")"
 usage() {
   echo "$0 <target>"
   echo
+  echo "This starts a Container specified by the first argument. Example:"
+  echo "icinga-build-docker centos/7:x86_64"
+  echo
+  echo "The Container will then run icinga-build-package as CMD"
+  echo "Important: This script needs to run in the directory that contains the build information for the package"
 }
 
+if [[ "$1" = "--help" ]]; then
+    usage
+    exit 0
+fi
+
 if [ -n "$1" ]; then
   TARGET="$1"
   shift
 else
   echo "Target is missing!" >&2
+  usage
   exit 1
 fi