Docker using huge amount of swap memory on Ubuntu 15.04, why?

I am suffering from excessive memory usage, currently amounting to ~940 MB physical and ~1 GB swap memory, on my Ubuntu 15.04 x64 Docker (1.9.1-cs2, build 4ade326) host. According to top, Docker is responsible for the consumption of about 780 MB of swap memory. How can I figure out why Docker is using so much swap?

I am mainly running a Node.js Docker container, which doesn't seem to consume that much memory. The memory usage is also rising over a few hours, as it's quite modest after booting up.

It's not a super satisfying answer, but enabling memory and swap accounting in the kernel seems to have reined in the excessive memory usage.