rkt downloads and extracts every docker image layer

[aquark]

This is a copy of #3831 which was prematurely closed. I have the exact same issue but I'm just copying the original issue text for convenience:

Environment

rkt Version: 1.29.0
appc Version: 0.8.11
Go Version: go1.8.3
Go OS/Arch: linux/amd64
Features: -TPM +SDJOURNAL
--
Linux 4.11.0-14-generic x86_64
--
NAME="Ubuntu"
VERSION="16.04.3 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.3 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial
--
systemd 229
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN

What did you do?

rkt run or rkt fetch docker:// image that has multiple layers after a update to a smaller layer.

What did you expect to see?

rkt download only changed layer and extract that.

What did you see instead?

rkt downloads ALL layers and re-extracts all of them to build aci.

I have base images that are 1 gig in size and having these layers downloaded and extracted every time a smaller upper layer is changed can take a very long time. Make dev testing with rkt frustrating. Maybe I'm missing something.