remove dependency to /proc/self/cgroup

/proc/self/cgroup does not contain the required information anymore

nginx_proxy.py

@@ -172,22 +172,9 @@ def get_pid():
 
 
 def get_docker_id():
-    """This function extracts the container's id from /proc/self/cgroup."""
-    id = ''
-    with open('/proc/self/cgroup', 'r') as file:
-        lines = file.read().split('\n')
-        for line in lines:
-            index = line.find('docker-')
-            if index != -1:
-                id = line[index+7:-6]
-                break
-            index = line.find('docker/')
-            if index != -1:
-                id = line[index+7:]
-                break
-    if id == '':
-        logging.error('could not determine container\'s id!')
-    return id
+    """This function extracts the container's id from /etc/hostname."""
+    with open('/etc/hostname', 'r') as file:
+        return file.read().strip()
 
 
 def get_listen_ips():