Prepare Docker image for Pixelfed

Summary:
This image is built on the top of the nginx/php7-fpm image,
battle-tested for Laravel applications with Auth Grove and
the notifications center.

Based on rDGROVE image at fc9168438227.

Ref T1481.

Test Plan: Deploy container based on this image to Equatower.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1481

Differential Revision: https://devcentral.nasqueron.org/D2023