Add deployment hooks to GitLab CI config

Add deployment hooks to the GitLab CI config (before I forget how it's done).

This would deploy all commits to the branch develop to the staging environment, and the main branch to the production environment.

Refs

• #58 (closed)
• https://whetstone.privatestorage.io/privatestorage/privatestorageops/-/issues/273
• https://whetstone.privatestorage.io/privatestorage/privatestorageops/-/issues/323