Add initial desired README.md

Dockerfile

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+FROM busybox
+ADD testfile.txt .
+RUN rm testfile.txt && mkdir app && echo "hello" > /app/othertestfile.txt
+

README.md

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+# Docker Artifact
+
+> Use docker image labels to identify which image layer contains a particular file, enabling retrieving just that file (well, the layer the file is in) from the image without needing to download the whole image.
+
+## Setup
+
+### Prerequisites
+
+Requires the `jq` and `curl` programs to be installed on your PATH.
+
+### Installation
+
+Download `docker-artifact.sh` file from this repository and install it at `~/.docker/cli-plugins/docker-artifact` (note the lack of `.sh` suffix) with execute permissions. Validate correct installation by observing the `artifact` command listed in `docker help`.
+
+-OR-
+
+Run the following command in your shell:
+
+```bash
+mkdir -p ~/.docker/cli-plugins && \
+  curl https://raw.githubusercontent.com/infogulch/docker-artifact/master/docker-artifact.sh > ~/.docker/cli-plugins/docker-artifact && \
+  chmod +x ~/.docker/cli-plugins/docker-artifact && \
+  docker help | grep artifact > /dev/null && echo "Docker artifact install succeeded!" || echo "Docker artifact install failed :("
+```
+
+## Usage
+
+`docker artifact label [image] [file-path-1] [file-path-2] ...`
+
+Adds labels to an existing image enabling the `download` command below to pull just the layers that contain the file paths specified above.
+
+Note: `download` must download the whole layer; to optimize for artifact download size, add the target files to the image in a separate layer.
+
+`docker artifact download [image] [file-path-1] [file-path-2]`
+
+Downloads the image layers associated with the file paths specified and extracts them into the current directory.
+
+## See also
+
+Related to [timwillfixit's original `docker-artifact`](https://github.com/tomwillfixit/docker-artifact) in spirit, though not in history. The primary difference is that this uses a more precice strategy to search for files.
+

docker-artifact.sh

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+set -e
+
+verbose=1
+image="$1"
+export searchpath="$2"
+export tarfile="$(mktemp)-$image.tar"
+
+# Save image tar to temp file
+[ $verbose ] && >&2 echo "Exporting image '$image' to temp file '$tarfile'..."
+docker image save "$image" -o "$tarfile"
+
+# Set up cleanup to not leave image behind
+function onexit() {
+  [ $verbose ] && >&2 echo "Cleaning up exported image '$tarfile'"
+  rm "$tarfile"
+}
+trap onexit EXIT
+
+# Extract manifest and config file contents
+[ $verbose ] && >&2 echo "Collecting metadata from image..."
+manifest=$(tar -xf "$tarfile" -x manifest.json -O | jq)
+config_file=$(echo "$manifest" | jq -r '.[0].Config')
+config=$(tar -f "$tarfile" -x "$config_file" -O | jq)
+
+# Combine manifest.json and config json to build a map from tar layer directory to layer id sha
+export idmap=$(echo "$manifest" "$config" | jq -sr '[ [ .[0][0].Layers, .[1].rootfs.diff_ids ] | transpose[] | { (.[0]): .[1] } ] | reduce .[] as $x ({}; . * $x)')
+
+# Search each layer for a file matching $searchpath
+# TODO also search for related whiteout files that start with `.wh.`
+[ $verbose ] && >&2 echo "Searching layers for '$searchpath'..."
+found=$(echo "$manifest" | jq '.[0].Layers[]' | xargs -I {} sh -c 'digest=$(echo "$idmap" | jq -r ".[\"{}\"]"); tar -f "$tarfile" -x {} -O | tar -t | sed s_^_/_ | grep -wx "$searchpath" | xargs -I [] echo "[]=$digest"')
+
+# If more than one is found, then bail
+if [ $(echo "$found" | wc -l) -gt 1 ]; then
+	>&2 echo "Multiple matches found, aborting:"
+	>&2 echo "$found"
+	exit 2
+fi
+
+# print out labels to add during image rebuild
+labels=$(echo "$found" | sed 's_^.*$_--label "\0"_' | paste -d' ')
+
+echo "$labels"
+
+
+[ $verbose ] && >&2 echo "Done!"
+exit
+

testfile.txt

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+hi