tests: Verify site-specific DH params feature works correctly

This addition requires usage of `DEFAULT_HOST` on containers tested to ensure they don't accidentally use `web2` as their default fallback (due to no SNI / `-servername` requested in openssl queries), otherwise they would be testing against the incorrect DH params response.

They could alternatively request an FQDN explicitly as well, instead of relying on implicit fallback/default server selection behaviour.

---

`web2.nginx-proxy.tld.dhparam.pem` is a copy of `ffdhe2048.pem`.

test/test_ssl/test_dhparam.yml

@@ -6,12 +6,27 @@ web5:
     WEB_PORTS: "85"
     VIRTUAL_HOST: "web5.nginx-proxy.tld"
 
+# Intended for testing with `dh-file` container.
+# VIRTUAL_HOST is paired with site-specific DH param file.
+# DEFAULT_HOST is required to avoid defaulting to web2,
+# if not specifying FQDN (`-servername`) in openssl queries.
+web2:
+  image: web
+  expose:
+    - "85"
+  environment:
+    WEB_PORTS: "85"
+    VIRTUAL_HOST: "web2.nginx-proxy.tld"
+
+
 # sut - System Under Test
 # `docker.sock` required for functionality
 # `certs` required to enable HTTPS via template
 with_default_group:
   container_name: dh-default
   image: &img-nginxproxy nginxproxy/nginx-proxy:test
+  environment: &env-common
+    - &default-host DEFAULT_HOST=web5.nginx-proxy.tld
   volumes: &vols-common
     - &docker-sock /var/run/docker.sock:/tmp/docker.sock:ro
     - &nginx-certs ./certs:/etc/nginx/certs:ro
@@ -20,6 +35,7 @@ with_alternative_group:
   container_name: dh-env
   environment:
     - DHPARAM_BITS=3072
+    - *default-host
   image: *img-nginxproxy
   volumes: *vols-common
 
@@ -27,12 +43,14 @@ with_invalid_group:
   container_name: invalid-group-1024
   environment:
     - DHPARAM_BITS=1024
+    - *default-host
   image: *img-nginxproxy
   volumes: *vols-common
 
 with_custom_file:
   container_name: dh-file
   image: *img-nginxproxy
+  environment: *env-common
   volumes:
     - *docker-sock
     - *nginx-certs
@@ -42,6 +60,7 @@ with_skip:
   container_name: dh-skip
   environment:
     - DHPARAM_SKIP=true
+    - *default-host
   image: *img-nginxproxy
   volumes: *vols-common
 
@@ -49,5 +68,6 @@ with_skip_backward:
   container_name: dh-skip-backward
   environment:
     - DHPARAM_GENERATION=false
+    - *default-host
   image: *img-nginxproxy
   volumes: *vols-common