mdBook can recognize
SUMMARY.md but cannot
summary.md on a GitHub Actions runner (Ubuntu VM).
On the other hand, both
summary.md are acceptable on my local macOS machine.
The case-sensitivity between the GitHub Actions runner and my local environment was different.
My local macOS machine has been formatted with case-insensitive mode, so the local building has passed and the remote building failed.
- GitHub Actions Ubuntu: Case sensitive
- My local macOS PC: Case insensitive
We can't offer another image at this time since it would affect overall capacity of Mac and be costly to maintain.