Minor cosmetic changes + clarify that user must exist when using <user>

doc/manual/docker-build.md

@@ -106,7 +106,8 @@ Here's an example:
   The archive formats have the advantage that file permission can be preserved better (since the copying is 
   independent from the underlying files systems), but might triggers internal bugs from the Maven assembler (as 
   it has been in #171)
-* **user** can be used to specify the user and group under which the files should be added. It has the general format 
+* **user** can be used to specify the user and group under which the files should be added. The user must be already exist in 
+  the base image. It has the general format 
   `user[:group[:run-user]]`. The user and group can be given either as numeric user- and group-id or as names. The group 
   id is optional. If a third part is given, then the build changes to user `root` before changing the ownerships, 
   changes the ownerships and then change to user `run-user` which is then used for the final command to execute. This feature

samples/data-jolokia-demo/pom.xml

@@ -298,6 +298,7 @@
                     <postExec>ls -l /maven</postExec>
                   </watch>
                 </image>
+
               </images>
             </configuration>
           </plugin>

src/main/java/org/jolokia/docker/maven/assembly/DockerFileBuilder.java

@@ -52,8 +52,7 @@ public class DockerFileBuilder {
     // list of RUN Commands to run along with image build see issue #191 on github
     private List<String> runCmds = new ArrayList<>();
 
-// environment
-
+    // environment
     private Map<String,String> envEntries = new HashMap<>();
 
     // image labels