Add our own Gemnasium DB Mirror

[zjrgov]

Story

As a DevTools customer's pipeline,
I would like to access a Gemnasium DB through egress proxy,
So that I can scan dependencies with Gemnasium.

Solution

• Add a Gemnasium DB someplace accessible from glr-workers, e.g. our GDG instance.
• Come up with a process to keep that G-DB updated automatically.
• Set GEMNASIUM_DB_REMOTE_URL to override the default URL (which is on gitlab.com).

Acceptance Criteria

• Gemnasium tests past without adding anything (gitlab.com) to the custom egress allowlist.
• Gemnasium DB is updated automatically.