begin command with docker source fails as non-root user

[danieldreier]

How to Reproduce

core@ip-10-224-6-13 /tmp $ acbuild begin docker://centos
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
Downloading sha256:da71393503e [=============================] 70.6 MB / 70.6 MB
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
begin: error extracting tarball
core@ip-10-224-6-13 /tmp $ sudo acbuild begin docker://centos
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
Downloading sha256:da71393503e [=============================] 70.6 MB / 70.6 MB
Downloading sha256:a3ed95caeb0 [=============================]       32 B / 32 B
core@ip-10-224-6-13 /tmp $

Versions

acbuild v0.3.1:

core@ip-10-224-6-13 /tmp $ acbuild version
acbuild version v0.3.1
appc version 0.8.4

CoreOS 1010.5.0:

core@ip-10-224-6-13 /tmp $ cat /etc/os-release
NAME=CoreOS
ID=coreos
VERSION=1010.5.0
VERSION_ID=1010.5.0
BUILD_ID=2016-05-26-2225
PRETTY_NAME="CoreOS 1010.5.0 (MoreOS)"
ANSI_COLOR="1;32"
HOME_URL="https://coreos.com/"
BUG_REPORT_URL="https://github.com/coreos/bugs/issues"

@dgonyeo and I discussed this on IRC, and he said he reproduced it as well. The execution context is CoreOS stable (1010.5.0) with no cloud-config or other post-provisioning configuration changes, aside from downloading the acbuild v0.3.1 release tarball from github and installing acbuild from there.