Move required files but 'nginx.tmpl' into a local 'app' folder and copy the
folder content into the image.
'nginx.tmpl' should be moved as well, but this is a breaking change for
configuration with a separate 'docker-gen' container.
- While the anonymous VOLUME can be dropped from Dockerfile, the path needs to be valid at run-time, might as well ensure it's available by creating the dhparam folder at build.
- Generation logic no longer necessary, dropped.
- Standardized RFC 7919 groups added (2048, 3072, 4096), with 4096-bit remaining the default size. The DH logic can live in the entrypoint script as well.
- Third-party supplied pre-generated DH params removed as they're not considered trustworthy compared to RFC 7919 groups.
