Challenge 2: Explain the Size Reduction — Possible Solution
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The builder stage's filesystem contains everything needed to PRODUCE
the build output: the full source tree, dev-only dependencies (test
frameworks, TypeScript's compiler, linters), any build tool caches,
and the compiled output itself, all sitting alongside each other. Most
of that is only relevant DURING the build process — once the compiled
dist/ folder exists, the TypeScript compiler that produced it and the
raw .ts source files it compiled from serve no further purpose for
actually RUNNING the application.

Copying the ENTIRE builder stage would carry all of that dead weight
into the final image — every dev dependency, every source file, every
build tool — none of which the running app ever touches, but all of
which still consumes disk space and, more importantly, represents
additional software (and therefore additional potential
vulnerabilities) sitting in a production container for no functional
benefit.

Copying ONLY /app/dist (the compiled output) and the production
node_modules (the packages actually required at runtime, not
devDependencies used only during build/test) means the final image
contains exactly what's needed to run the application and nothing
else. This is precisely why the size difference is so large in
practice — the excluded material (full source, dev tooling, build
caches) is often the majority of what a typical single-stage image
actually contains, even though none of it does anything once the app
is running.