Vendorize boto.
s3cat: a tool that dumps an S3 object to stdout.
Copy JVM to .jdk/
Set JAVA_TOOL_OPTIONS just like the standard JVM buildpacks do.
s3cat: --etag lets you retrieve the object's ETag.
Cache JDK and validate the cached JDK based on ETag.
Compute SHA1 of the build directory's content.