[infra] flattening base-clang image

The image has lots of layers with lots of deleted files (sources and intermediates).
Flatten the image to reduce its size.

infra/base-images/Jenkinsfile

@@ -18,6 +18,16 @@
 
 def dockerOptions="--no-cache"
 
+// Flattens all intermediate layers of the image.
+// Usefull when image has lots of deleted files.
+def flattenDockerImage(image) {
+  def containerName = image.replaceAll('/', '_') + "-tmp";
+  sh "echo ${image} ${containerName}"
+  sh "docker rm -f ${containerName} || true"
+  sh "docker run --name ${containerName} ${image} /bin/true"
+  sh "docker export ${containerName} | docker import - ${image}"
+}
+
 node {
   git url: 'https://github.com/google/oss-fuzz/'
 
@@ -27,6 +37,8 @@ node {
   stage name: 'Build ossfuzz/base-clang', concurrency: 1
   sh "docker build $dockerOptions -t ossfuzz/base-clang infra/base-images/base-clang"
 
+  flattenDockerImage("ossfuzz/base-clang")
+  
   stage name: 'Build ossfuzz/base-libfuzzer', concurrency: 1
   sh "docker build $dockerOptions -t ossfuzz/base-libfuzzer infra/base-images/base-libfuzzer"