My first update to ubuntu 16 went fine, the second host’s upgrade resulted in a read-only filesystem (seems to be a common enough issue.. I may simply move it to a new host as is recommended).
However, it seems that perhaps I never explicitly requested the install of the image-extras package which is required for AUFS. Since I ran the apt-get command @robbyoconnor suggested, all is well with the host/docker/discourse.