updates - apt-get upgrade: why is lxc-docker being kept back?

Why is lxc-docker being held back? I also notice that my web app, running on Docker, is quite slow. Any suggestions?

[lucas@production]/home/lucas$ sudo apt-get dist-upgrade
Reading package lists... Done

Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following package was automatically installed and is no longer required:
  linux-image-3.16.0-28-generic
Use 'apt-get autoremove' to remove it.
The following packages have been kept back:
  lxc-docker
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
[lucas@production]/home/lucas$ sudo apt-get upgrade

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following package was automatically installed and is no longer required:
  linux-image-3.16.0-28-generic
Use 'apt-get autoremove' to remove it.
The following packages have been kept back:
  lxc-docker
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

Note that I have already run sudo apt-get dist-upgrade, which does not resolve this issue. Thus, I don't think my questions is a duplicate of this questions

You can upgrade lxc-docker specifically:

sudo apt-get install lxc-docker

That should override whatever holds back the upgrade (barring manually held packages, which we excluded in this case).