This isn’t really useful for frameworks like rails, since there’s nothing to “compile” there. Most rails docker images will just include the runtime and a few C dependencies, which you need to run the app.
Pulling down gems is a bit of a compilation, which could benefit, unless you're already installing gems into a volume you include in the Docker container via docker compose etc. Additionally, what it does compile can be fairly slow (like nokogiri).
There are however temporary files being downloaded for the apt installation, and while in this case it's simple enough to remove them in one step that's by no means always the case. Depending on which gems you decide to rely on you may e.g. also end up with a full toolchain to build extensions and the like, so knowing the mechanism is worthwhile.
How would you go about copying something you installed from apt in a build container?
Say `apt install build-essential libvips` from the OP, it's not obvious to me what files libvps is adding. I suppose there's probably an incantation for that? What about something that installs a binary? Seems like a pain to chase down everything that's arbitrarily touched by an apt install, am I missing some tooling that would tame that pain?
It's a pain, hence for apt as long as it's the same packages, just cleaning is probably fine. But e.g. build-essential is there to handle building extensions pulled in by gems, and that isn't necessary in the actual container if you bring over the files built and/or installed by rubgygems, so the set of packages can be quite different.
Run "dpkg -L libvips" to find the files belonging to that package. This doesn't cover what's changed in post install hooks, but for most docker-relevant things, it's good enough.
