Proposal: Allow setting ulimits for containers

Add option to set ulimit settings for containers.
Currently the container inherits from the daemon's ulimit settings.

Support for this was added in libcontainer here docker/libcontainer@7ce34f58cbd4884634a0f1f82c4aaab1168ea0de
I know this wasn't done so far, but perhaps the "What's new" lines should include a link to the API / CLI section(s) that explain how they can be used, ie [see `--default-ulimit`](#). It's sometimes difficult to find the corresponding change.