ci: align Mergify queue checks with main (#902)

## Summary
- switch the Mergify base branch condition from master to main
- require both docker-test matrix checks in the dependabot merge queue

## Verification
- docker compose -f .devcontainer/docker-compose.yml up -d
- docker exec devcontainer-app-1 bash -lc 'cd /workspace && make setup
&& make ready'

## Notes
- this slice changes only .mergify.yml and was rebuilt from main after
#901 merged
- the Dev Container mount for linked worktrees does not preserve
in-container git metadata, so verification was executed from /workspace
without relying on git commands inside the container

Author: gildesmarais

.mergify.yml

@@ -4,10 +4,11 @@ queue_rules:
       - author=dependabot[bot]
     merge_conditions:
       - author=dependabot[bot]
-      - status-success=docker-test
+      - check-success=docker-test (false)
+      - check-success=docker-test (true)
       - status-success=hadolint
       - status-success=ruby
-      - base=master
+      - base=main
     merge_method: squash
 
 pull_request_rules: