wip: rootfs

BUILD

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+load("//src/undocker:rules.bzl", "rootfs")
 
 go_library(
     name = "go_default_library",

rules.bzl

@@ -0,0 +1,31 @@
+def _rootfs_impl(ctx):
+    ctx.actions.run(
+        executable =  ctx.files._undocker,
+        arguments = [
+            ctx.attr.src,
+            ctx.output,
+        ],
+    )
+
+rootfs = rule(
+    doc = "Generate a rootfs from a docker container image",
+    implementation = _rootfs_impl,
+    attrs = {
+        "src": attr.label(
+            doc = "Input container tarball",
+            mandatory = True,
+            allow_single_file = [".tar"],
+        ),
+        "out": attr.output(
+            doc = "Output rootfs tarball",
+            mandatory = True,
+        ),
+        "_undocker": attr.label(
+            doc = "undocker cli; private and may not be overridden",
+            cfg = "host",
+            default = Label("//src/undocker:undocker"),
+            allow_single_file = True,
+            executable = True,
+        ),
+    },
+)