Recompress slim layers to gzip before pushing#33
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richturner merged 1 commit intomainfrom May 8, 2026
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`docker save` writes uncompressed layer tarballs (`application/vnd.oci.image.layer.v1.tar`). Pushed as-is, the slim image landed in the registry as a single ~2.56 GB uncompressed blob — larger than the un-slimmed base — so the slim toolkit's size win was being thrown away on the way out. Insert a `regctl image mod --layer-compress gzip --replace` step between import and digest computation so layers are rewritten to `application/vnd.oci.image.layer.v1.tar+gzip` before the push. The digest must be computed after the mod step since recompressing layers changes the manifest digest.
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Summary
Follow-up to #32. After it landed, the slim variant on Docker Hub was larger than the un-slimmed base (
develop-slim2.56 GB amd64 / 2.48 GB arm64 vsdevelop1.91 GB / 1.97 GB).Inspecting the manifests showed why: the slim image had a single layer with media type
application/vnd.oci.image.layer.v1.tar— uncompressed — whiledevelop's layers were...tar.gzip.docker savealways writes uncompressed layer tarballs, and the import → copy pipeline shipped them to the registry as-is, throwing away the slim toolkit's actual size win.Insert a
regctl image mod --layer-compress gzip --replacestep between import and digest computation. It rewrites layer blobs toapplication/vnd.oci.image.layer.v1.tar+gzipin place. The digest is captured after the mod, since recompressing layers changes layer digests → manifest digest.Verified locally that
image mod --layer-compress gzipdoes what's advertised: round-tripping alpine through--layer-compress none(8.0 MB uncompressed) →--layer-compress gzip(3.6 MB) recovers the gzipped form, and the layer media type updates correctly.regctl image modis tagged EXPERIMENTAL by upstream — meaning the flag interface may evolve in future versions, not that the command is unstable. Sincerelease: v0.11.3is pinned, the surface is locked.