Add docs about minimum size requirements for shm_size

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

doc/docker/README.md

@@ -600,3 +600,30 @@ sudo setfacl -mR default:group:docker:rwx $GITLAB_HOME
 The default group is `docker`. If you changed the group, be sure to update your
 commands.
+
+### /dev/shm mount not having enough space in Docker container
+
+GitLab comes with a Prometheus metrics endpoint at `/-/metrics` to expose a
+variety of statistics on the health and performance of GitLab. The files
+required for this gets written to a temporary file system (like `/run` or
+`/dev/shm`).
+
+By default, Docker allocates 64Mb to the shared memory directory (mounted at
+`/dev/shm`). This is insufficient to hold all the Prometheus metrics related
+files generated, and will generate error logs like the following:
+
+```plaintext
+writing value to /dev/shm/gitlab/sidekiq/gauge_all_sidekiq_0-1.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/gauge_all_sidekiq_0-1.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/gauge_all_sidekiq_0-1.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/histogram_sidekiq_0-0.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/histogram_sidekiq_0-0.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/histogram_sidekiq_0-0.db failed with unmapped file
+writing value to /dev/shm/gitlab/sidekiq/histogram_sidekiq_0-0.db failed with unmapped file
+```
+
+Other than disabling the Prometheus Metrics from the Admin page, the recommended
+solution to fix this problem is to increase the size of shm to at least 256Mb.
+If using `docker run`, this can be done by passing the flag `--shm-size 256m`.
+If using a `docker-compose.yml` file, the `shm_size` key can be used for this
+purpose.