Ensure env is passed to DockerLayer actions so xzcat works

This is required to fully fix #611.

--
Change-Id: Iffeab2a80a02f2e1208637fe85195d0c2862e637
Reviewed-on: https://bazel-review.googlesource.com/#/c/3492
MOS_MIGRATED_REVID=120724036

tools/build_defs/docker/docker.bzl

@@ -109,6 +109,7 @@ def _build_layer(ctx):
       arguments = args,
       inputs = ctx.files.files + ctx.files.tars + ctx.files.debs,
       outputs = [layer],
+      use_default_shell_env=True,
       mnemonic="DockerLayer"
       )
   return layer