This fixes the following error when running the "docker-system" test:
In ice-9/ftw.scm:
553:30 1 (_ #<directory stream db6660>)
In unknown file:
0 (readdir #<directory stream db6660>)
ERROR: In procedure readdir:
In procedure readdir: Cannot allocate memory
* gnu/tests/docker.scm (%test-docker-system): Bump image building VM memory
size to 1024 MiB.