Move the sha256 tool into its own directory, as it isn't specific to docker_build.

PiperOrigin-RevId: 153508081
diff --git a/tools/BUILD b/tools/BUILD
index 3923626..97b160e 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -12,6 +12,7 @@
         "//tools/build_defs/apple:srcs",
         "//tools/build_defs/apple/test:srcs",
         "//tools/build_defs/docker:srcs",
+        "//tools/build_defs/hash:srcs",
         "//tools/build_defs/pkg:srcs",
         "//tools/build_defs/repo:srcs",
         "//tools/build_rules:srcs",