feat: add explicit docker cleanup script retention

[exKAZUu]

Summary

• update cleanup.sh to accept --keep-scripts
• keep plain bash ./bash/cleanup.sh as the final cleanup path that removes the copied ./bash helper directory
• remove unused ASDF helper scripts from generated Docker helpers

Why

• downstream Dockerfiles sometimes need apt cleanup before later generated helpers such as configure-yarn.sh
• the cleanup behavior should be visible in the Dockerfile command instead of relying on WB_KEEP_DOCKER_SCRIPTS
• unused helper scripts add noise to generated dist/bash output

Testing

• bash -n packages/wb/docker/bash/cleanup.sh
• rg "install-asdf\\.sh|prepare-asdf.*\\.sh|prepare-asdf\\.sh" packages/wb packages/wbfy -g '!node_modules' -g '!dist'
• checked local WillBooster / WillBoosterLab Dockerfiles for helper usage
• yarn verify

[gemini-code-assist]

Code Review

This pull request introduces a cleanup.sh script designed to optimize Docker images by purging unnecessary packages and removing build-time scripts. The review feedback highlights that the apt-get purge command should be wrapped in a conditional block to respect the --keep-scripts flag, preventing the removal of tools like curl and wget when they are still needed for subsequent build steps. Additionally, the feedback suggests including git in the purge list and refactoring the brittle directory name check to ensure the script directory is reliably removed even if renamed.