feat: Replace generated DH params with RFC 7919 standardized DHE groups

- 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.

docker-entrypoint.sh

@@ -1,6 +1,37 @@
 #!/bin/bash
 set -e
 
+function _setup_dhparam() {
+	echo 'Setting up DH Parameters..'
+
+	# DH params will be supplied for nginx here:
+	DHPARAM_FILE='/etc/nginx/dhparam/dhparam.pem'
+
+	# DH params may be provided by the user (rarely necessary),
+	# or use an existing pre-generated group from RFC7919, defaulting to 4096-bit:
+	if [[ -f ${DHPARAM_FILE} ]]
+	then
+		echo 'Warning: A custom dhparam.pem file was provided. Best practice is to use standardized RFC7919 DHE groups instead.' >&2
+	else
+		# ENV DHPARAM_BITS - Defines which RFC7919 DHE group to use (default: 4096-bit):
+		local FFDHE_GROUP="${DHPARAM_BITS:-4096}"
+		# RFC7919 groups are defined here:
+		# https://datatracker.ietf.org/doc/html/rfc7919#appendix-A
+		local RFC7919_DHPARAM_FILE="/app/dhparam/ffdhe${FFDHE_GROUP}.pem"
+
+		# Only the following pre-generated sizes are supported,
+		# emit an error and kill the container if provided an invalid value:
+		if [[ ! ${DHPARAM_BITS} =~ ^(2048|3072|4096)$ ]]
+		then
+			echo "ERROR: Unsupported DHPARAM_BITS size: ${DHPARAM_BITS}, use 2048, 3072, or 4096 (default)." >&2
+			exit 1
+		fi
+
+		# Provide the DH params file to nginx:
+		cp "${RFC7919_DHPARAM_FILE}" "${DHPARAM_FILE}"
+	fi
+}
+
 # Warn if the DOCKER_HOST socket does not exist
 if [[ $DOCKER_HOST = unix://* ]]; then
 	socket_file=${DOCKER_HOST#unix://}
@@ -14,8 +45,7 @@ if [[ $DOCKER_HOST = unix://* ]]; then
 	fi
 fi
 
-# Generate dhparam file if required
-/app/generate-dhparam.sh
+_setup_dhparam
 
 # Compute the DNS resolvers for use in the templates - if the IP contains ":", it's IPv6 and must be enclosed in []
 RESOLVERS=$(awk '$1 == "nameserver" {print ($2 ~ ":")? "["$2"]": $2}' ORS=' ' /etc/resolv.conf | sed 's/ *$//g'); export RESOLVERS