pkg: Add extend for extending partitions

This was split out from pkg/format into its own package.
It has the ability to extend ext4, btrfs and xfs partitions.

Signed-off-by: Dave Tucker <dt@docker.com>

pkg/extend/Dockerfile

@@ -0,0 +1,33 @@
+FROM linuxkit/alpine:488aa6f5dd2d8121a3c5c5c7a1ecf97c424b96ac 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 \
+    e2fsprogs \
+    e2fsprogs-extra \
+    btrfs-progs \
+    xfsprogs \
+    xfsprogs-extra \
+    musl \
+    sfdisk \
+    util-linux \
+    && true
+RUN rm -rf /out/etc/apk /out/lib/apk /out/var/cache
+
+FROM linuxkit/alpine:488aa6f5dd2d8121a3c5c5c7a1ecf97c424b96ac AS build
+
+RUN apk add --no-cache go musl-dev
+ENV GOPATH=/go PATH=$PATH:/go/bin
+
+COPY extend.go /go/src/extend/
+RUN go-compile.sh /go/src/extend
+
+FROM scratch
+ENTRYPOINT []
+CMD []
+WORKDIR /
+COPY --from=mirror /out/ /
+COPY --from=build /go/bin/extend usr/bin/extend
+CMD ["/usr/bin/extend"]
+LABEL org.mobyproject.config='{"binds": ["/dev:/dev"], "capabilities": ["CAP_SYS_ADMIN", "CAP_MKNOD"], "net": "new", "ipc": "new"}'