Use DOCKER_LOCKEDMEMORY to set ulimit -l before starting Docker. 

## Reason:

Running memory heavy applications, such as elastic search, benefit from having locked memory. This can be achieved by using ulimit -l  

## Additions:
Added DOCKER_LOCKEDMEMORY as a var that will allow people to change the settings. This works just like DOCKER_NOFILE, which allows to change number of open file descriptors.