CMake: add a way to skip trying to set up docker on Linux

I keep getting this warning because docker-compose is not installed.

Change-Id: Id8e48e8f498c4a029619fffd172932bd86ed03ba
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

cmake/QtTestHelpers.cmake

@@ -147,6 +147,9 @@ function(qt_internal_setup_docker_test_fixture name)
     if (NOT CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux")
       return()
     endif()
+    if (QT_SKIP_DOCKER_COMPOSE)
+      return()
+    endif()
 
     set(QT_TEST_SERVER_LIST ${ARGN})
     set(DNSDOMAIN test-net.qt.local)