- re-enable ability to use a random container name

src/main/asciidoc/inc/image/_naming.adoc

@@ -50,6 +50,9 @@ When specifying the container name pattern the following placeholders can be use
 | *%i*
 | An index which is incremented if a container has already been created. With this parameter it is easily possible to have multiple, similar containers. See the example below for more details.
 
+| *%r*
+| A random container name choosen by the docker engine.
+
 |===
 
 You can combine the placeholders in any combination and will be resolved during `docker:start`, `docker:stop` and `docker:watch`.

src/main/java/io/fabric8/maven/docker/util/ContainerNamingUtil.java

@@ -35,15 +35,15 @@ public static String formatContainerName(final ImageConfiguration image,
         String containerNamePattern = extractContainerNamePattern(image, defaultContainerNamePattern);
         Set<String> existingContainersNames = extractContainerNames(existingContainers);
 
-        final String partiallyApplied =
-            replacePlaceholders(
-                containerNamePattern,
-                image.getName(),
-                image.getAlias(),
-                buildTimestamp);
-
-
-        if (partiallyApplied.contains(INDEX_PLACEHOLDER)) {
+        // don't replace if we want a random container name
+        String partiallyApplied = containerNamePattern == null ? null :
+                replacePlaceholders(
+                        containerNamePattern,
+                        image.getName(),
+                        image.getAlias(),
+                        buildTimestamp);
+
+        if (partiallyApplied != null && partiallyApplied.contains(INDEX_PLACEHOLDER)) {
             for (long i = 1; i < Long.MAX_VALUE; i++) {
                 final String withIndexApplied = partiallyApplied.replaceAll(INDEX_PLACEHOLDER, String.valueOf(i));
                 if (!existingContainersNames.contains(withIndexApplied)) {
@@ -143,16 +143,27 @@ private static String extractContainerNamePattern(ImageConfiguration image, Stri
         RunImageConfiguration runConfig = image.getRunConfiguration();
         if (runConfig != null) {
             if (runConfig.getContainerNamePattern() != null) {
-                return runConfig.getContainerNamePattern();
+                return getContainerNamingPattern(runConfig.getContainerNamePattern());
             }
             if (runConfig.getNamingStrategy() == RunImageConfiguration.NamingStrategy.alias) {
                 return "%a";
             }
         }
         return defaultContainerNamePattern != null ? defaultContainerNamePattern : DEFAULT_CONTAINER_NAME_PATTERN;
     }
-
+    
     private static String cleanImageName(final String imageName) {
         return new ImageName(imageName).getSimpleName().replaceAll("[^a-zA-Z0-9_.-]+", "_");
     }
+
+    private static String getContainerNamingPattern(String pattern) {
+        if (pattern.contains("%r")) {
+            if (pattern.length() > 2) {
+                throw new IllegalArgumentException("Invalid use of container naming pattern '%r'"); 
+            }                       
+            return null;
+        }
+
+        return pattern;
+    }
 }

src/test/java/io/fabric8/maven/docker/util/ContainerNamingUtilTest.java

@@ -40,6 +40,16 @@ public void testAlias() {
                                 Collections.emptySet()));
     }
 
+    @Test
+    public void testRandom() {
+        Assert.assertEquals(null,
+                ContainerNamingUtil.formatContainerName(
+                    imageConfiguration("jolokia/jolokia_demo","nameAlias", "%r"),
+                    null,
+                    new Date(123456),
+                    Collections.emptySet()));
+    }
+
     @Test
     public void testTimestamp() {
         Assert.assertEquals("123456",