feat: Pull invalid nginx configuration when the nginx configs fail to validate

- expose an nginx:validate command that can be used to show validation information
- properly reference the invalid nginx config, rather than tell the user that another app's nginx config is invalid when deploying their own app
- allow a user to cleanup bad nginx config files out of band when they are blocking a deploy _without_ requiring knowing where that nginx config is

Note that this may have issues with generated nginx.conf files that depend on other apps or other parts of the nginx config. For now, this is acceptable, though it is something that needs to be field tested.

Closes #3162

tests/unit/40_nginx-vhosts_2.bats

@@ -106,3 +106,50 @@ assert_error_log() {
   echo "status: $status"
   assert_failure
 }
+
+@test "(nginx-vhosts) nginx:validate" {
+  deploy_app
+  run /bin/bash -c "dokku nginx:validate"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:validate $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  echo "invalid config" > "/home/dokku/${TEST_APP}/nginx.conf"
+
+  run /bin/bash -c "dokku nginx:validate"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+
+  run /bin/bash -c "dokku nginx:validate $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+
+  run /bin/bash -c "dokku nginx:validate --clean"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:validate"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  echo "invalid config" > "/home/dokku/${TEST_APP}/nginx.conf"
+
+  run /bin/bash -c "dokku nginx:validate $TEST_APP --clean"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:validate"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}