apko erofs with compression support via mkfs.erofs

docs/apko_file.md

@@ -274,3 +274,19 @@ It contains the following children:
  - `budget`: The number of additional layers apko will use for layering.
 
 See [layering.md](layering.md) for more information.
+
+### Format (experimental)
+
+`format` selects the on-wire layer payload format:
+
+ - `tar` (default): gzip-compressed tar layers (`application/vnd.oci.image.layer.v1.tar+gzip`).
+ - `erofs`: EROFS filesystem images (`application/vnd.erofs`), per the draft [erofs/erofs-image-spec](https://github.com/erofs/erofs-image-spec). Written by a pure-Go writer with no internal compression.
+ - `erofs+ALGO[,level=N]`: compressed EROFS via `mkfs.erofs` (`erofs-utils`); `ALGO` is one of `zstd`, `lz4`, `lz4hc`, or `deflate`. The optional `level=N` is passed through to `mkfs.erofs -z<ALGO>,level=N`.
+
+EROFS layers advertise `erofs` in the image config's `os.features` so consumers that do not implement the spec can identify and skip them. Compressed layers also set `org.erofs.uncompressed-digest` on the layer descriptor to the SHA-256 of the equivalent uncompressed image, which equals the layer's DiffID in `rootfs.diff_ids`.
+
+`format` may also be selected on the command line with `--format=…` on `apko build` and `apko publish`. The CLI flag overrides whatever is in the config file.
+
+**Status:** EROFS support is experimental and tracks the spec PR at https://github.com/erofs/erofs-image-spec/pull/1; media types and annotations may change before the spec reaches a stable release. Both single-layer and multi-layer (`layering`) builds are supported. Multi-layer builds emit each non-final layer with `org.erofs.role=overlay-lower` per spec §3.8; the final layer carries no role. dm-verity is not implemented. Compressed (`erofs+ALGO`) variants require `mkfs.erofs` on `PATH`.
+
+See [erofs.md](erofs.md) for a step-by-step guide to building, inspecting, mounting, and pulling EROFS images.