fix(coverage): skip docker integration tests on the KVM runner

Docker integration tests are already covered by the integration-docker
job on ubuntu-latest. On the KVM runner the Firecracker TAP/nftables
pool conflicts with Docker networking, causing those tests to hang
and the coverage job to never complete.

Add SKIP_DOCKER_TESTS env-var support to docker_available() and set
it for the integration phase of coverage.sh so only Firecracker
integration tests run there.

tests/_docker.py

@@ -2,12 +2,15 @@
 
 from __future__ import annotations
 
+import os
 import shutil
 import subprocess
 import unittest
 
 
 def docker_available() -> bool:
+    if os.environ.get("SKIP_DOCKER_TESTS"):
+        return False
     if shutil.which("docker") is None:
         return False
     try: