Add xvfb-init.sh to docker context

.dockerignore

@@ -1 +1,2 @@
 *
+!tools/xvfb-init.sh

script/cibuild

@@ -60,9 +60,9 @@ def main():
       deps += LINUX_DEPS_NO_ARM
     execute(['sudo', 'apt-get', 'install'] + deps)
 
- if PLATFORM == 'linux':
-  os.environ['DISPLAY'] = ':99.0'
-  execute(['sh', '-e', '/etc/init.d/xvfb', 'start'])
+  if PLATFORM == 'linux':
+    os.environ['DISPLAY'] = ':99.0'
+    execute(['sh', '-e', '/etc/init.d/xvfb', 'start'])
 
   # CI's npm is not reliable.
   npm = 'npm.cmd' if PLATFORM == 'win32' else 'npm'