Add container_gids column to DB groups table

The UID/GID values in the groups table will be applied to all containers created in sessions owned by that group or owned by a user who is associated with that group.

### groups table

`container_gids`: nullable array of integer field

JIRA Issue: BA-676

Start by locating the groups table definition and its database migration in the repository. Confirm how nullable array fields are represented, then verify that the groups table includes a nullable integer array named container_gids and that existing database checks or tests pass.

Written by the indexing model from the issue text.