WORKSPACE: bump rules_docker

This fixes a bazel_tools incompatibility with Bazel 3.x.

Change-Id: I73a5beafe03b1d8e68a1fee794961146a2f10c52
diff --git a/WORKSPACE b/WORKSPACE
index 95cecf9..a7b37af 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -104,9 +104,9 @@
 
 http_archive(
     name = "io_bazel_rules_docker",
-    sha256 = "dc97fccceacd4c6be14e800b2a00693d5e8d07f69ee187babfd04a80a9f8e250",
-    strip_prefix = "rules_docker-0.14.1",
-    urls = ["https://github.com/bazelbuild/rules_docker/releases/download/v0.14.1/rules_docker-v0.14.1.tar.gz"],
+    sha256 = "1698624e878b0607052ae6131aa216d45ebb63871ec497f26c67455b34119c80",
+    strip_prefix = "rules_docker-0.15.0",
+    urls = ["https://github.com/bazelbuild/rules_docker/releases/download/v0.15.0/rules_docker-v0.15.0.tar.gz"],
 )
 
 load("@io_bazel_rules_docker//toolchains/docker:toolchain.bzl", docker_toolchain_configure = "toolchain_configure")