Earlier quoted context omitted.
> But those problems are independent of build frontends: you could solve them once for both buildpacks and Dockerfiles. You can build an image with both technologies, but that's not the key to the argument. The key here is every Dockerfile is unique and potentially quite different from any other Dockerfile. Small differences in layer order and layer contents multiply to very large inefficiencies at scale. The way you…
> Small differences in layer order and layer contents multiply to very large inefficiencies at scale. Can you provide an example of how layer order can cause an issue?
FROM nodejs
COPY /mycode /app
RUN npm install /app
An NPM buildpack can signal that there's nothing to change, allowing the overall lifecycle to skip re-building that layer.
There's also the problem of efficient composition. Suppose I have this:
RUN wget https://example.com/popular-shell-script.sh && \
go get https://git.example.com/something-else@abc123 && \
./popular-shell-script.sh && \
rm ./popular-shell-script
RUN go get https://git.example.com/something-else@abc123
But I don't get to do that, because the layers have different overall contents and different digests. Buildpacks don't get you all the way to a file-centric approach, but because they follow a repeatable, controlled pattern of selecting the contents and order of layers, they greatly improve layer reuse between many images.