Devmapper insufficient thin pool free data blocks.

[John Ralls <jralls ceridwen us>]

I just pushed a very small MacOS change 
(https://gitlab.gnome.org/GNOME/gtk/commit/828f634d5c08b9c39626dd1512ec7a1af4800f3d) and got a CI failure 
(https://gitlab.gnome.org/GNOME/gtk/-/jobs/31820).

I compared that with the previous push to gtk-3-22, which passed. The failing log begins:
"Running with gitlab-runner 10.7.0 (7c273476)
  on progress.gnome.org f23948e6
Using Docker executor with image registry.gitlab.gnome.org/gnome/gtk/gtk-3-22:v2 ...
ERROR: Preparation failed: Error response from daemon: devmapper: Thin Pool has 155512 free data blocks which 
is less than minimum required 163840 free data blocks. Create more free space in thin pool or use 
dm.min_free_space option to change behavior"

While the passing one begins:
"Running with gitlab-runner 10.7.1 (b9bba623)
  on jenkins.gnome.org 9b338874
Using Docker executor with image registry.gitlab.gnome.org/gnome/gtk/gtk-3-22:v2 ...
Pulling docker image registry.gitlab.gnome.org/gnome/gtk/gtk-3-22:v2 ..."

So a different gitlab-runner version on a different host. Is that the root of the problem? Is there anything 
I can do about it?

Regards,
John Ralls