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

blimp/engine/BUILD.gn

Index: blimp/engine/BUILD.gn
diff --git a/blimp/engine/BUILD.gn b/blimp/engine/BUILD.gn
index 4c8fa0de6d54a59264c870dbb6b0439a0d01ae51..5b8c310b3c91e3e6ea74c583c6cd9351c6963133 100644
--- a/blimp/engine/BUILD.gn
+++ b/blimp/engine/BUILD.gn
@@ -536,6 +536,7 @@ if (is_linux) {
     # These form the arguments to the script.
     _rebased_out_dir = rebase_path(root_out_dir)
     _rebased_dockerfile = rebase_path("//blimp/engine/Dockerfile")
+    _rebased_dockerfile_base = rebase_path("//blimp/tools/Dockerfile.base")
     _rebased_startup_script = rebase_path("//blimp/engine/start_engine.sh")
     _bundle = "$root_out_dir/blimp_engine_bundle.tar.gz"
 
@@ -548,6 +549,7 @@ if (is_linux) {
     sources = [
       _manifest,
       _rebased_dockerfile,
+      _rebased_dockerfile_base,
       _rebased_startup_script,
     ]
     outputs = [
@@ -560,6 +562,7 @@ if (is_linux) {
       _rebased_out_dir,
       "--filelist",
       _rebased_dockerfile,
+      _rebased_dockerfile_base,
       _rebased_startup_script,
       "--manifest",
       _rebased_manifest,