tools: Add a mkimage variant to build a non-bootable ISO.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

tools/mkimage-iso/Dockerfile

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+FROM linuxkit/alpine:daed76b8f1d28cdeeee215a95b9671c682a405dc AS mirror
+RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
+RUN apk add --no-cache --initdb -p /out \
+    alpine-baselayout \
+    busybox \
+    cdrkit \
+    libarchive-tools \
+    && true
+RUN mv /out/etc/apk/repositories.upstream /out/etc/apk/repositories
+
+FROM scratch
+WORKDIR /
+COPY --from=mirror /out/ /
+COPY . .
+ENTRYPOINT [ "/make-iso" ]

tools/mkimage-iso/build.yml

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+image: mkimage-iso
+arches:
+  - amd64

tools/mkimage-iso/make-iso

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+#!/bin/sh
+
+set -e
+
+mkdir -p /tmp/iso
+cd /tmp/iso
+
+# input is a tarball of filesystem on stdin
+# output is an iso on stdout
+
+# extract. BSD tar auto recognises compression, unlike GNU tar
+# only if stdin is a tty, if so need files volume mounted...
+[ -t 0 ] || bsdtar xzf -
+
+genisoimage -o ../linuxkit.iso -l -J -R \
+		-joliet-long -input-charset utf8 \
+                -V LinuxKit .
+cat ../linuxkit.iso