The build step wrote to /src/target inside the image layer, so any source change recompiled the whole workspace from zero, including all dependencies.
Mount the target directory as a cache mount and copy the binaries to /out within the same step, since the contents of a mount are not part of the committed layer. The final stage then copies from /out.
With the cache mount, a source change only recompiles the crates that changed, and an interrupted build resumes instead of starting over. The syntax works with podman/buildah and with Docker's BuildKit builder.
