tools: Add mkimage-squashfs

This utility image takes a tarball as input and places the
contents into a read-only, compressed squashfs filesystem
which is produced on stdout.

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

tools/mkimage-squashfs/Dockerfile

@@ -0,0 +1,12 @@
+FROM linuxkit/alpine:f3cd219615428b2bd943411723eb28875275fae7
+
+RUN \
+  apk update && apk upgrade && \
+  apk add --no-cache \
+  libarchive-tools \
+  squashfs-tools \
+  && true
+
+COPY . .
+
+ENTRYPOINT [ "/make-squashfs" ]

tools/mkimage-squashfs/build.yml

@@ -0,0 +1 @@
+image: mkimage-squashfs

tools/mkimage-squashfs/make-squashfs

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -e
+
+mkdir -p /tmp/rootfs
+cd /tmp/rootfs
+
+# input is a tarball of filesystem on stdin with the root filesytem
+# output is a squashfs image 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 -
+cd /tmp
+
+# we want everything except the final result to stderr
+(
+    exec 1>&2;
+
+    mksquashfs rootfs ./rootfs.img
+)
+cat rootfs.img