Docker process is eating a lot of swap on Ubuntu 15.04

I ram running Docker version 1.9.1-cs2, build 4ade326 on Ubuntu 15.04 x64 (DigitalOcean) via Tutum. The docker process itself is using a ton of swap memory (as verified via VmSwap in /proc/<docker PID>/status), slowly growing over about a day, until about 780 MB. The system has 1 GB of RAM and 2 GB of swap in total.

Can anyone please explain to me why docker is eating all this swap? It’s bringing my system to its knees.

For reference, you can see my ServerFault question on the matter.

$ docker info
Containers: 12
Images: 207
Server Version: 1.9.1-cs2
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 231
 Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.19.0-31-generic
Operating System: Ubuntu 15.04
CPUs: 1
Total Memory: 993.1 MiB
Name: staging.muzhack.com
ID: YX6B:6FOP:VNZI:Y5EK:QKD7:CEKE:UAWE:CQW7:YVJ7:XHEH:KM36:WTL7