Where do I need to copy files in a multistage image build?

I'm learning multistage build to get a small image size.
However, I'm struggling with "where do I copy files?"

Indeed, considering the Dockerfile below:

FROM --platform=linux/arm64/v8 debian:11.6-slim as builder

WORKDIR /app

RUN apt update
RUN apt install curl unzip -y
RUN apt-get -y install sqlite3
RUN apt-get -y install imagemagick
RUN curl https://bun.sh/install | bash

COPY package.json .
COPY bun.lockb .

RUN /root/.bun/bin/bun install --production

##############################

FROM --platform=linux/arm64/v8 gcr.io/distroless/base

WORKDIR /app

COPY --from=builder /root/.bun/bin/bun bun
COPY --from=builder /app/node_modules node_modules
# COPY --from=builder     <===== SQLITE
# COPY --from=builder     <===== CURL
# COPY --from=builder     <===== IMAGEMAGICK

CMD ["./bun", "index.ts"]

EXPOSE 3000

What and where do I need to copy to get SQLite3, curl and imagemagick work in my distroless image? And whatever the application is, how do I know where to save it?

BTW, if there is another way to shrink my image size, let me know. Thanks.

You can't really do this.

ImageMagick in particular includes multiple binaries and has several shared-library dependencies. You'd need to identify what all of these things are an COPY all of them into the final image. Looking at the Debian package page can give you some idea; follow all of the "dep:" dependency links, and click through to the "list of files" links at the bottom of each package's page. This will eventually include the libc6 and libgcc-s1 low-level shared-library packages, and if any of these programs are implemented as shell scripts you'll need a shell too; at which point you'll essentially have a complete functioning Linux environment.

SQLite is another interesting case in that you probably don't need the sqlite3 command-line tool, but something in your node_modules directory will need the corresponding libsqlite.so.3 shared library and its dependencies. So you'd also have to identify these cases in your application's dependencies with native extensions.

You don't say where the space in your image is actually going, and your image seems to be missing the actual application code. My generic advice would be to split the build into two stages, where the first stage builds the application and the second runs it. I wouldn't worry about using an Alpine or distroless image – the base distribution probably isn't where your space is going and you'll hit compatibility problems like this. Do try to only install the shared libraries you need in the final stage, not ...-dev pacakges and not the command-line tools.