The downside of this is the same as the upside: it stuffs all that logic into one layer. If the result of your setup script changes at all, then the cache for that entire layer and all later layers are busted. This may or may not be what you want.

As a concrete example... if your setup.sh were:

  #!/bin/bash
  ./update_static_assets.sh
  ./install_libraries.sh
  ./clone_application_repo.sh

then any time a static asset is updated, a library is changed, or your application code changes, the digest of the Docker layer for `RUN ./setup.sh` will change. Your team will then have to re-download the result of all three of those sub-scripts next time they `docker pull`.

However, if you found that static assets changed less often than libraries, which changed less often than your application code, then splitting setup.sh into three correspondingly-ordered `RUN` statements would put the result of each sub-script its own layer. Then, if just your application code changed, you and your team wouldn't need to re-download the library and static asset layers.

I do this for all of the CI images I maintain. Additionally, it leaves evidence of the setup in the container itself. Usually I have a couple of these scripts (installing distro-provided deps, building each group of other deps, etc.).