pkg/modprobe from linuxkit/alpine base, using established multistage build onto scratch pattern

Signed-off-by: Isaac Rodman <isaac@eyz.us>

docs/kernels.md

@@ -38,6 +38,15 @@ In summary, LinuxKit offers a choice of the following kernels:
 - [linuxkit/kernel-fedora](https://hub.docker.com/r/linuxkit/kernel-fedora/): Selected Fedora kernels.
 
 
+## Loading kernel modules
+
+Most kernel modules are autoloaded with `mdev` but if you need to `modprobe` a module manually you can use the `modprobe` package in the `onboot` section like this:
+```
+  - name: modprobe
+    image: linuxkit/modprobe:<hash>
+    command: ["modprobe", "-a", "iscsi_tcp", "dm_multipath"]
+```
+
 ## Compiling external kernel modules
 
 This section describes how to build external (out-of-tree) kernel

pkg/modprobe/Dockerfile

@@ -0,0 +1,13 @@
+FROM linuxkit/alpine:87a0cd10449d72f374f950004467737dbf440630 AS mirror
+
+RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
+RUN apk add --no-cache --initdb -p /out \
+    busybox
+RUN rm -rf /out/etc/apk /out/lib/apk /out/var/cache
+
+FROM scratch
+ENTRYPOINT []
+CMD []
+WORKDIR /
+COPY --from=mirror /out/ /
+LABEL org.mobyproject.config='{"binds": ["/lib/modules:/lib/modules", "/sys:/sys"], "capabilities": ["CAP_SYS_MODULE"]}'

pkg/modprobe/Makefile

@@ -0,0 +1,3 @@
+IMAGE=modprobe
+
+include ../package.mk