Make container swap limit configurable independently of memory limit

[goruklu]

Summary

The OCI config templates currently hard-code the cgroup memory.memsw.limit_in_bytes (swap) to the same value as memory.limit_in_bytes, making it impossible to configure swap independently of the memory limit.

"memory": {
    "limit": {{MEM_LIMIT}},
    "swap": {{MEM_LIMIT}},   <-- always tied to memLimit
    "swappiness": 60
},

Proposed Change

New optional swapLimit spec field

Add a new top-level swapLimit field to the Dobby JSON spec. When present it sets the cgroup swap limit independently of memLimit. When absent the behaviour is unchanged — swap defaults to the same value as memLimit (i.e. no additional swap beyond the memory limit).

{
    "memLimit": 2998272,
    "swapLimit": 5996544
}

The kernel requires memory.memsw.limit_in_bytes >= memory.limit_in_bytes, so an error is returned if swapLimit is set lower than memLimit.

Files to change

File	Change
bundle/lib/source/DobbySpecConfig.cpp	New MEM_SWAP ctemplate constant; JSON_FLAG_SWAPLIMIT flag; new processSwapLimit() processor registered in the field map; step-5 default sets MEM_SWAP = memLimit when swapLimit is absent
bundle/lib/include/DobbySpecConfig.h	Declaration of processSwapLimit()
bundle/lib/source/templates/OciConfigJson1.0.2-dobby.template	"swap": {{MEM_SWAP}} (was {{MEM_LIMIT}})
bundle/lib/source/templates/OciConfigJsonVM1.0.2-dobby.template	Same template fix

Backward compatibility

Fully backward compatible. Existing specs that only specify memLimit produce identical OCI output to before.