sessions have 2 hour limit, but several sessions were running for more than that limit

[chunhualiao]

For example, "Up 14 hours" shown in below.

 docker ps
CONTAINER ID        IMAGE                             COMMAND                  CREATED              STATUS              PORTS                                           NAMES
4ab59621a096        franela/dind                      "dockerd-entrypoint.…"   About a minute ago   Up About a minute   2375/tcp                                        bn0ovbuv_bn0ovbuveh5g00dgvaj0
f16f317b1db5        franela/dind                      "dockerd-entrypoint.…"   2 hours ago          Up 2 hours          2375/tcp                                        bn0nhvuv_bn0ni06veh5g00dgvai0
fcb090a15c7f        franela/dind                      "dockerd-entrypoint.…"   11 hours ago         Up 11 hours         2375/tcp                                        bn0fkimv_bn0fkimveh5g00dgvac0
5462aa6442d0        franela/dind                      "dockerd-entrypoint.…"   14 hours ago         Up 14 hours         2375/tcp                                        bn0cckev_bn0cckmveh5g00dgva40
788fc8716ffd        franela/play-with-docker:latest   "./play-with-docker …"   6 weeks ago          Up 45 hours         0.0.0.0:5010->3000/tcp, 0.0.0.0:443->3001/tcp   pwd
b3dc8a11c5d0        jekyll/jekyll:3                   "/usr/jekyll/bin/ent…"   3 months ago         Up 2 weeks          35729/tcp, 0.0.0.0:5020->4000/tcp               compiler-classroom_jekyll_1

[vec4]

I've noticed this seems to happen when the user starts a sandbox instance from a tutorial, and then leaves the page, or even just refreshes it, before the timer is up. It seems the timer gets lost and the spawned instance continues to run.