--continue work for all tasks. How do I specify in task definition or command line to ignore failure of only the specified task?
If the task implementation provides a way to do so, you have to use that facility, like ignoreFailures for tasks of type Test.
For tasks that do not provide such functionality, you are probably lost.
Besides maybe using --continue and then having a task execution listener that would fail hardly if it is not one of the tasks to be ignored. But I have no idea whether this would actually work.