docker: support retaining at least a few files in backup retention

[liuxhit]

Describe the pull request

A clear and concise description of what the pull request is about, i.e. what problem should be fixed?

Link to the issue: #7488

Checklist

• [ x] I agree to follow the Code of Conduct by submitting this pull request.
• [ x] I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code or have provided the test plan.

Test plan

Verified in my personal deployment environment:

1. modify source code
2. do build: docker build --network host --build-arg https_proxy=$https_proxy --rm --pull -f Dockerfile -t gogs_selfbuild:test .
3. do upgrade: docker-compose down && sleep 3 && docker-compose up -d
4. do verify:

• a. check service uptime and healthy status
• b. exec into docker and check crontab content in /etc/crontabs/git:

docker exec -it gogs bash
cat /etc/crontabs/git

• c. manually execute the backup-rotator.sh and check files in BACKUP_PATH:

docker exec -it gogs bash
su - git
ls -l /backup/
/bin/sh /app/gogs/docker/runtime/backup-rotator.sh '/backup' '7'
ls -l /backup/
/bin/sh /app/gogs/docker/runtime/backup-rotator.sh '/backup' '7'
/bin/sh /app/gogs/docker/runtime/backup-rotator.sh '/backup' '7'
ls -l /backup/