Add test to check that apt-get didn't remove any package

tests/constants.py

@@ -0,0 +1 @@
+DOCKER_IMAGE = 'readthedocs/build:ubuntu20'

tests/test_apt.py

@@ -0,0 +1,13 @@
+import pytest
+
+from .constants import DOCKER_IMAGE
+from .utils import run_command_in_container
+
+
+
+def test_apt_dont_remove_packages():
+    command ='grep "Remove:" /var/log/apt/history.log'
+    expected_output = ''
+
+    cmd_output = run_command_in_container(DOCKER_IMAGE, command)
+    assert cmd_output == expected_output

tests/test_versions.py

@@ -1,6 +1,6 @@
 import pytest
-from docker import APIClient
 
+from .constants import DOCKER_IMAGE
 from .utils import run_command_in_container
 
 
@@ -13,5 +13,5 @@
     ]
 )
 def test_command_versions_image_ubuntu20(command, expected_output):
-    cmd_output = run_command_in_container('readthedocs/build:ubuntu20', command)
+    cmd_output = run_command_in_container(DOCKER_IMAGE, command)
     assert cmd_output == expected_output