Override build user+host+timestamp

[bmwiedemann]

Override build user+host+timestamp
to make kata-linux-container package builds reproducible.

See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/ for the definition of the variable.
SOURCE_DATE_EPOCH is unset atm in the .spec file case because no changelog is there to allow rpm to parse the date from it.

Fixes part of issue #394

This PR was done while working on reproducible builds for openSUSE.

[jodh-intel]

Thanks @bmwiedemann!!

To keep our CI happy, please could you add a Signed-off-by: comment in your commit (see https://github.com/kata-containers/community/blob/master/CONTRIBUTING.md#general-format).

[jodh-intel]

Thanks very much @bmwiedemann!

lgtm

[jodh-intel]

/test

[devimc]

lgtm

[jodh-intel]

CI is failing with:

[Fail] Update number of CPUs Update CPU constraint [It] should have 1 CPUs 
/tmp/jenkins/workspace/kata-containers-packaging-ubuntu-18-04-PR/go/src/github.com/kata-containers/tests/integration/docker/cpu_test.go:299

/cc @devimc - this seems to be an intermittent failure.

[devimc]

@jodh-intel yes, and I think the problem is in the agent, I'm thinking that 1 second is not enough time to wait for new vCPUs in nested environments, see https://github.com/kata-containers/agent/blob/master/grpc.go#L69-L71

[marcov]

lgtm, just a comment about timestamp.

[egernst]

@bmwiedemann this is great, thank you. Does this completely address reproducible builds for linux-container ?

[bmwiedemann]

@egernst it does for the binaries in the rpm. For fully bit-identical rpm packages, there would need to be a %changelog entry that can be used by rpmbuild to override file mtime and "Build Time" rpm header values.

And then it would be possible that if you add more variations like Debian (varying build-path, umask, timezone, locale) then you could get variations in binaries again.
But those do not matter in the context of OBS builds, which is what I mostly care about.

[jodh-intel]

Nice - thanks @bmwiedemann.

/retest

[jodh-intel]

18.04 CI fell over with:

not ok 1 Check CPU constraints
# (in test file k8s-cpu-ns.bats, line 35)
#   `[ $total_cpus_container -eq $total_cpus ]' failed

/retest as I can't see this PR would cause that...