feat: Bring back ability to skip default DH params

Adds back the ability to avoid using DH params, provided no file was explicitly supplied.

This used to be `DHPARAM_GENERATION=false`, the equivalent is now `DHPARAM_SKIP=1` (default 0). Previous name was no longer appropriate.

Ensures that if a user has explicitly provided their own dhparam file to still output a warning instead of the skip message, since `DHPARAM_SKIP=1` doesn't disable the support in nginx.

test/test_ssl/test_dhparam.yml

@@ -38,4 +38,13 @@ with_custom_file:
   volumes:
     - /var/run/docker.sock:/tmp/docker.sock:ro
     - ./certs:/etc/nginx/certs:ro
-    - ../lib/ssl/dhparam.pem:/etc/nginx/dhparam/dhparam.pem:ro
+    - ../lib/ssl/dhparam.pem:/etc/nginx/dhparam/dhparam.pem:ro
+
+with_skip:
+  image: nginxproxy/nginx-proxy:test
+  container_name: dh-skip
+  environment:
+    - DHPARAM_SKIP=1
+  volumes:
+    - /var/run/docker.sock:/tmp/docker.sock:ro
+    - ./certs:/etc/nginx/certs:ro