Gunicorn is replaced with nginx-unit

We now serve Netbox with an nginx-unit instance instead of Gunicorn.
This allows us to get rid of the extra Nginx container because Unit is
also serving the static files. The static files are now collected at container
buildtime instead of every startup.

docker/nginx-unit.json

@@ -0,0 +1,40 @@
+{
+    "listeners": {
+        "*:8080": {
+            "pass": "routes"
+        }
+    },
+
+    "routes": [
+        {
+            "match": {
+                "uri": "/static/*"
+            },
+            "action": {
+                "share": "/opt/netbox/netbox"
+            }
+        },
+
+        {
+            "action": {
+                "pass": "applications/netbox"
+            }
+        }
+    ],
+
+    "applications": {
+        "netbox": {
+            "type": "python 3",
+            "path": "/opt/netbox/netbox/",
+            "module": "netbox.wsgi",
+            "home": "/opt/netbox/venv",
+            "processes": {
+                "max": 4,
+                "spare": 1,
+                "idle_timeout": 120
+            }
+        }
+    },
+
+    "access_log": "/dev/stdout"
+}