Proposal: add TEST label

[shaded-enmity]

The purpose of the TEST label is for the image authors to be able to specify a self-contained check to figure out whether the image can run correctly on the given host.

Use case:

FROM fedora:22
LABEL TEST='sh -c "([ -c /dev/kvm ] && [ \"$(stat -c \"%a\" /dev/kvm)\" -eq "666\" ]) || (echo \"Bad KVM device\"; exit 1)"'

$ atomic test my-kvm-image
ERROR: Bad KVM device

[fatherlinux]

OH, I like that. That could help solve my system call problem too:

#39