feat: var BACKUP_SERVER to tag server as backup

nginx.tmpl

@@ -317,8 +317,13 @@ upstream {{ $vpath.upstream }} {
             {{- $args = dict "container" $container "path" $path "port" $port }}
             {{- template "container_port" $args }}
             {{- if $ip }}
-                {{- $servers = add1 $servers }}
+                {{- $backup := parseBool (coalesce $container.Env.BACKUP_SERVER "false") }}
+                {{- if not $backup }}
+                    {{- $servers = add1 $servers }}
     server {{ $ip }}:{{ $args.port }};
+                {{- else }}
+    server {{ $ip }}:{{ $args.port }} backup;
+                {{- end }}
             {{- end }}
         {{- end }}
     {{- end }}

test/test_backup-server/test_backup-server.py

@@ -0,0 +1,14 @@
+import pytest
+
+
+def test_web1_is_backup(docker_compose, nginxproxy):
+    for i in range(1, 10):
+        r = nginxproxy.get("http://web1.nginx-proxy.tld/port")
+        assert r.status_code == 200
+        assert r.text == "answer from port 91\n"
+
+def test_web2_is_production(docker_compose, nginxproxy):
+    for i in range(1, 10):
+        r = nginxproxy.get("http://web2.nginx-proxy.tld/port")
+        assert r.status_code == 200
+        assert r.text == "answer from port 82\n"

test/test_backup-server/test_backup-server.yml

@@ -0,0 +1,44 @@
+version: "2"
+
+services:
+  web1:
+    image: web
+    expose:
+      - "81"
+    environment:
+      WEB_PORTS: 81
+      VIRTUAL_HOST: web1.nginx-proxy.tld
+    network_mode: "none"
+
+  web1_backup:
+    image: web
+    expose:
+      - "91"
+    environment:
+      WEB_PORTS: 91
+      VIRTUAL_HOST: web1.nginx-proxy.tld
+      BACKUP_SERVER: "true"
+
+  web2:
+    image: web
+    expose:
+      - "82"
+    environment:
+      WEB_PORTS: 82
+      VIRTUAL_HOST: web2.nginx-proxy.tld
+
+  web2_backup:
+    image: web
+    expose:
+      - "92"
+    environment:
+      WEB_PORTS: 92
+      VIRTUAL_HOST: web2.nginx-proxy.tld
+      BACKUP_SERVER: "true"
+
+  sut:
+    image: nginxproxy/nginx-proxy:test
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+    environment:
+      HTTPS_METHOD: nohttps