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" ]