Clear entrypoint when setting command (#77)

If a builder image has an entrypoint (say via inheritance from it's base builder image) then in order for the snowdrop buildpack lib to function correctly, we must wipe the entrypoint when setting the command, else we'll invoke the wrong thing.

client/src/main/java/dev/snowdrop/buildpack/docker/ContainerUtils.java

@@ -107,6 +107,7 @@ private static String createContainerInternal(DockerClient dc, String imageRefer
 
     if (command != null) {
       ccc.withCmd(command);
+      ccc.withEntrypoint("");
     }
 
     if (network!=null){