Don't defer docker whitelist check to another thread

This check is not time-consuming so it can be performed in the calling
thread instead of being deferred to another thread.

Actually, this is a workaround for integration tests failing on Windows.
For some reason if database was used in another thread (as it happened
before this change) the unit test cleanup would fail because "The
golem.db is used by another process and cannot be removed".

golem/envs/docker/cpu.py

@@ -8,7 +8,7 @@
     NamedTuple, Tuple, Iterator, Union, Iterable
 
 from docker.errors import APIError
-from twisted.internet.defer import Deferred, inlineCallbacks
+from twisted.internet.defer import Deferred, inlineCallbacks, succeed
 from twisted.internet.threads import deferToThread
 from urllib3.contrib.pyopenssl import WrappedSocket
 
@@ -580,13 +580,14 @@ def install_prerequisites(self, prerequisites: Prerequisites) -> Deferred:
                              f"is in invalid state: '{self._status}'")
         logger.info("Preparing prerequisites...")
 
+        if not Whitelist.is_whitelisted(prerequisites.image):
+            logger.info(
+                "Docker image '%s' is not whitelisted.",
+                prerequisites.image,
+            )
+            return succeed(False)
+
         def _prepare():
-            if not Whitelist.is_whitelisted(prerequisites.image):
-                logger.info(
-                    "Docker image '%s' is not whitelisted.",
-                    prerequisites.image,
-                )
-                return False
             try:
                 client = local_client()
                 client.pull(