docker is not needed to run tests.

--- pytest_skip_markers-1.5.2/requirements/tests.txt.orig
+++ pytest_skip_markers-1.5.2/requirements/tests.txt
@@ -1,4 +1,3 @@
-docker
 pytest-subtests
 pyfakefs==4.4.0; python_version == '3.5'
 pyfakefs; python_version > '3.5'