Create docker group on Dwellers

Summary:
On Docker development server, allow to manage
Docker as a non-root user for operations members.

That avoids to prepend each command with sudo.

Ref T1595.

Test Plan: docker ps on Dwellers

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1595

Differential Revision: https://devcentral.nasqueron.org/D2221