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History and Manifest

Reading an Image's History and Manifest

docker history lists an image's build steps from the newest down, with the instruction that made each one and the size it added. It is the quickest way to find out why someone else's image is large:

The build history of l3-layersShell
docker history l3-layers:1.0 \
  --format 'table {{.ID}}\t{{.CreatedSince}}\t{{.Size}}\t{{.CreatedBy}}'
Output
IMAGE          CREATED         SIZE      CREATED BY
effc91e93bde   5 minutes ago   0B        CMD ["cat" "/hello.txt"]
<missing>      5 minutes ago   8.19kB    COPY hello.txt /hello.txt # buildkit
<missing>      5 minutes ago   4.1kB     RUN /bin/sh -c rm /big.bin # buildkit
<missing>      5 minutes ago   21MB      RUN /bin/sh -c dd if=/dev/urandom of=/big.bi…
<missing>      7 days ago      0B        CMD ["/bin/sh"]
<missing>      7 days ago      9.08MB    ADD alpine-minirootfs-3.24.2-x86_64.tar.gz /…

The bottom two rows come from alpine:3's own build, which added its root filesystem a week earlier. <missing> does not signal a problem: BuildKit 10,294 does not create a separate image for each step, so only the final row has an ID. The history travels with the image config, so --no-trunc also reveals any secret passed on a command line or in ARG (Secrets in Images).

The manifest is the registry's view: which config and layer blobs make up the image for one platform. For a multi-platform tag, the index lists one manifest per architecture, which is how docker pull chooses the right one:

The platforms behind node:24-slimShell
docker buildx imagetools inspect node:24-slim | grep Platform | head -4
Output
  Platform:    linux/amd64
  Platform:    unknown/unknown
  Platform:    linux/arm64/v8
  Platform:    unknown/unknown

Each unknown/unknown entry is an attestation (provenance or SBOM data) attached to the manifest before it, not an image; Container Security reads them. The free tools dive 54,614 (https://github.com/wagoodman/dive 54,614 ) and crane 4,061 (https://github.com/google/go-containerregistry 4,061 ) browse layers and manifests interactively and from scripts.