Create a Dockerfile.base and ensure it is included in build tarballs.

blimp/BUILD.gn

Index: blimp/BUILD.gn
diff --git a/blimp/BUILD.gn b/blimp/BUILD.gn
index cd1d0f5b87462bf1570ba34aef9f9b0d56470205..053551abcf692762f0bf7822af9888a51d1e1604 100644
--- a/blimp/BUILD.gn
+++ b/blimp/BUILD.gn
@@ -147,6 +147,7 @@ if (is_linux) {
     # These form the arguments to the script.
     _rebased_out_dir = rebase_path(root_out_dir)
     _rebased_dockerfile = rebase_path("//blimp/Dockerfile")
+    _rebased_dockerfile_base = rebase_path("//blimp/tools/Dockerfile.base")
     _bundle = "$root_out_dir/blimp_engine_env_tests_bundle.tar.gz"
 
     # Depending on this ensures that both the manifest is generated
@@ -158,6 +159,7 @@ if (is_linux) {
     sources = [
       _blimp_engine_env_tests_manifest,
       _rebased_dockerfile,
+      _rebased_dockerfile_base,
     ]
     outputs = [
       _bundle,
@@ -167,6 +169,7 @@ if (is_linux) {
       _rebased_out_dir,
       "--filelist",
       _rebased_dockerfile,
+      _rebased_dockerfile_base,
       "--manifest",
       _rebased_blimp_engine_env_tests_manifest,
       "--output",