That is correct, in fact it will build a version for any x86-64 CPU even before v2. For v2 specifically you can specify build argument --build-arg="RUSTFLAGS=\"-C target-cpu=x86-64-v2 -C target-feature=+aes\"" (I think, didn’t try it). You can find RUSTFLAGS build argument in corresponding Dockerfile.
See subspace/.github/workflows/snapshot-build.yml at c7e17117d2abb2ed8ef072f1dbbcdfe2be4f8acf · subspace/subspace · GitHub for examples of actual values we’re using (you can just fork the repo, add/change things and let CI build it for you automatically).
It worked but I had to run - minor change to the quotes.
docker buildx build --build-arg RUSTFLAGS="-C target-cpu=x86-64-v2 -C target-feature=+aes" --progress plain -t hakehardware/autonomys-legacy-farmer:gemini-3h-2024-jun-18 -f Dockerfile-farmer .