increase the size of /dev/shm in ci runners

To get reproducible builds for Arti (tpo/core/arti!69 (merged)), it's required to have a tmpfs of at least ~270MiB. The CI provides a tmpfs of 64MiB in /dev/shm, which is not enough, but it can be configured to be bigger (gilab doc, scroll to shm_size).
Currently builds are reproducible, in CI, but they rely on a bug in Docker to get a big enough tmpfs (/sys/fs/cgroup is mounted as a rw tmpfs without size limit), however this is not ideal, and will definitely get patched.

Could you please increase the size of /dev/shm in gitlab runners to something bigger than that (384 MiB for instance)?

cc @nickm