antlir2 as compared to other image build systems
General goals
antlir2 is tightly integrated with buck2 so that monorepo artifacts can
easily be built into images combined with upstream packages.
antlir2 makes it (almost) impossible for non-determinism to leak into image
builds (aside from badly behaved buck rules providing inputs to be installed).
vs docker build
docker build with a Dockerfile is something that many engineers are familiar
with - it provides a relatively easy to read description about how to build an
image.
Determinism
Dockerfiles almost always have a line like dnf update -y before installing
any packages. This means that every time you run docker build, you're getting
a live view of the upstream package repos.
While this is generally acceptable by many users, it makes it impossible to
retroactively rebuild an image as it would have been produced by an earlier
build. Forget about bisecting when a dependency changes!
antlir2 has an SCM-tracked snapshot of upstream package repositories, so that
builds are always perfectly reproducible on a given SCM
rev.
Caching
The unit of caching for a Dockerfile is each line. If a line changes (or any
of the lines preceding it change) it will be re-run on the next docker build.
antlir2's logical layering unit is an image.layer target. However, caching
is more intelligent within this level, making it more reasonable to mix package
installation with your own code that depends on those packages, since that
package installation will still be cached.
Building other artifacts
It's not very interesting to build an image without any of your own software in
it (if that's all you want, a prebuilt docker image is probably good enough).
To build your own code in a Dockerfile requires you to install any
build dependencies, then copy your source code, run a compiler, move the output
somewhere and then finally (if you don't forget) delete the build dependencies
and source code, leaving a clean image that you actually want to deploy.
In antlir2, you can use buck2 to build whatever you want, and install only
the artifacts that you want in the final image.