Allow nginx to access container_file_t certificates on Docker engines

Summary:
When upgrading CentOS 8, nginx lost capability to read Let's Encrypt
fullchain.pem and privkey.pem files, both with container_file_t context
as they are managed by a Certbot container.

This change updates the SELinux policy to allow this operation.

Ref T1592

Test Plan: Tested on Equatower

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1592

Differential Revision: https://devcentral.nasqueron.org/D2205