wip docker/alpine

BUILD

@@ -1,5 +1,7 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
 load("//src/undocker:rules.bzl", "rootfs")
+load("@io_bazel_rules_docker//container:container.bzl", "container_bundle",
+)
 
 go_library(
     name = "go_default_library",
@@ -18,3 +20,14 @@ go_binary(
     embed = [":go_default_library"],
     visibility = ["//visibility:public"],
 )
+
+container_bundle(
+    name="alpine",
+    images={"base": "@alpine//image"},
+)
+
+rootfs(
+    name="alpine-rootfs1",
+    src=":alpine.tar",
+    output="alpine-rootfs.tar",
+)

rules.bzl

@@ -1,22 +1,27 @@
 def _rootfs_impl(ctx):
+    output = ctx.outputs.output
     ctx.actions.run(
-        executable =  ctx.files._undocker,
+        outputs = [output],
+        executable = ctx.files._undocker[0],
         arguments = [
-            ctx.attr.src,
+            ctx.attr.src.path,
-            ctx.output,
+            output.path,
         ],
     )
 
 rootfs = rule(
     doc = "Generate a rootfs from a docker container image",
     implementation = _rootfs_impl,
+    outputs = {
+        "out": "%{name}.tar",
+    },
     attrs = {
         "src": attr.label(
             doc = "Input container tarball",
             mandatory = True,
             allow_single_file = [".tar"],
         ),
-        "out": attr.output(
+        "output": attr.output(
             doc = "Output rootfs tarball",
             mandatory = True,
         ),