Stop using DefaultInfo.runfiles (#138)

* Stop using DefaultInfo.runfiles

Use DefaultInfo.default_runfiles instead of DefaultInfo.runfiles, as DefaultInfo.runfiles is an undocumented API that is being completely eliminated in the current Bazel release (0.16.0).

* Stop using DefaultInfo.runfiles

Stop using DefaultInfo.runfiles in docker_config.bzl. Use DefaultInfo.default_runfiles instead.
diff --git a/container/rules/docker_toolchains.bzl b/container/rules/docker_toolchains.bzl
index b2533f1..2fba04e 100644
--- a/container/rules/docker_toolchains.bzl
+++ b/container/rules/docker_toolchains.bzl
@@ -176,8 +176,13 @@
     # Install tars and configure env, symlinks using the container_image rule.
     result = _container.image.implementation(ctx, base = new_base, symlinks = symlinks, env = env, tars = tars, files = files)
 
+    if hasattr(result.providers[1], "runfiles"):
+        result_runfiles = result.providers[1].runfiles
+    else:
+        result_runfiles = result.providers[1].default_runfiles
+
     return struct(
-        runfiles = result.providers[1].runfiles,
+        runfiles = result_runfiles,
         files = result.providers[1].files,
         container_parts = result.container_parts,
         tars = tars,
diff --git a/rules/docker_config.bzl b/rules/docker_config.bzl
index 76b5676..ca33340 100644
--- a/rules/docker_config.bzl
+++ b/rules/docker_config.bzl
@@ -280,7 +280,11 @@
     )
 
     # add to the runfiles the script to load image and (if needed) the repo_pkg_tar file
-    runfiles_list = result.providers[1].runfiles.files.to_list() + [ctx.outputs.load_image]
+    if hasattr(result.providers[1], "runfiles"):
+        result_runfiles = result.providers[1].runfiles
+    else:
+        result_runfiles = result.providers[1].default_runfiles
+    runfiles_list = result_runfiles.files.to_list() + [ctx.outputs.load_image]
     if ctx.attr.repo_pkg_tar:
         runfiles_list += ctx.files.repo_pkg_tar