Allow xpack.spaces.defaultSolution to be configured via docker (elastic#236570)

(cherry picked from commit 7e3586e)

Author: legrego

src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker

@@ -419,6 +419,7 @@ kibana_vars=(
     xpack.securitySolution.packagerTaskInterval
     xpack.securitySolution.prebuiltRulesPackageVersion
     xpack.spaces.maxSpaces
+    xpack.spaces.defaultSolution
     xpack.task_manager.capacity
     xpack.task_manager.claim_strategy
     xpack.task_manager.auto_calculate_default_ech_capacity