overlay: freeze atomic due to pa/atomic#1217
    
    A recent commit in projectatomic/atomic broke how `atomic uninstall`
    works for non-system containers.  This caused our automated sanity
    tests to start failing.
    
    The last known good commit of `atomic` is
    56c32aeca382f2c68e5078679b7585837cb13716, so let's freeze to that.