Improve error handling when Docker daemon is not running

Author: amotl

CHANGES.rst

@@ -6,6 +6,8 @@ pytest-mqtt changelog
 in progress
 ===========
 
+- Improve error handling when Docker daemon is not running. Thanks, @horta.
+
 
 2023-03-15 0.2.0
 ================

pytest_mqtt/mosquitto.py

@@ -55,6 +55,12 @@ def pull_image(self):
         docker_client.images.pull(image_name)
 
     def run(self):
+        docker_client = docker.from_env(version=self.docker_version)
+        docker_url = docker_client.api.base_url
+        try:
+            docker_client.ping()
+        except Exception:
+            raise ConnectionError(f"Cannot connect to the Docker daemon at {docker_url}. Is the docker daemon running?")
         self.pull_image()
         return super(Mosquitto, self).run()