Add feature to enable SSL/TLS site via env params

This feature allows nginx to export an HTTPS site in addition to (or redirected from) the HTTP site.

Note: This patch needs significant testing which I don't currently have the time to perform.  Since my initial testing, it has been heavily modified to conform to the syntax used on the sameersbn/docker-gitlab repository, and has not been fully tested with these changes.  It was also originally developed before the recent slew of changes to the sameersbn/docker-redmine repository.  Rather than hold on to it and need to rebase it yet again as the image evolves, I'm offering it as is in hopes that someone else can test it.

Although there's no mention in the README.md file of importing config files via a volume mount, I reused its parent directory to hold the certs directory with the intent of allowing a single volume mount to cover both scenarios.

I suggest testing with a volume mount of the certs to the default places as well as explicitly specifying the file locations via the SSL_*_PATH variables.  I would also test it with and without the DHPARAM file, and with all combinations of REDMINE_HTTPS and REDMINE_HTTPS_ONLY variables.

I borrowed heavily from the docker-gitlab repo's README verbiage, but removed much of the gitlab-specific certificate restrictions.  Please ensure what I copied and modified makes sense in the context of a redmine service.