erofs output (#4162)

* add erofs as output option

Signed-off-by: Avi Deitcher <avi@deitcher.net>

* unify nearly identical functions

Signed-off-by: Avi Deitcher <avi@deitcher.net>

---------

Signed-off-by: Avi Deitcher <avi@deitcher.net>

tools/mkimage-erofs/Dockerfile

@@ -0,0 +1,15 @@
+FROM linuxkit/alpine:6090baae063eb5023c9601966e88df831f789a70 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 \
+  libarchive-tools \
+  erofs-utils \
+  && true
+RUN mv /out/etc/apk/repositories.upstream /out/etc/apk/repositories
+
+FROM scratch
+WORKDIR /
+COPY --from=mirror /out/ /
+COPY . .
+ENTRYPOINT [ "/make-erofs" ]

tools/mkimage-erofs/build.yml

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

tools/mkimage-erofs/make-erofs

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+#!/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;
+
+    mkfs.erofs ./rootfs.img rootfs
+)
+cat rootfs.img