Add initial search prototype

Dockerfile

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

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