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.py

@@ -168,6 +168,16 @@ def test_custom_dhparam_is_supported(docker_compose):
     can_negotiate_dhe_ciphersuite(sut_container)
 
 
+def test_can_skip_dhparam(docker_compose):
+    container_name="dh-skip"
+    sut_container = docker_client.containers.get(container_name)
+    assert sut_container.status == "running"
+
+    assert_log_contains("Skipping Diffie-Hellman parameters setup.", container_name)
+
+    cannot_negotiate_dhe_ciphersuite(sut_container)
+
+
 def test_web5_https_works(docker_compose, nginxproxy):
     r = nginxproxy.get("https://web5.nginx-proxy.tld/port", allow_redirects=False)
     assert r.status_code == 200

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