Initial support for reproducible builds

[rn]

This PR adds initial support for reproducible linuxkit builds (wip #2813).

• Files created during the build have their timestamp fixed
• Slices/Arrays are sorted before being marshalled to JSON

Currently, the tar, tar-kernel-initrd, docker, and kernel+initrd output formats support reproducible builds and tests for tar and kernel+initrd are added with this PR.

[deitch]

This is rather nice!

Is there concern that the modtime now is just the timestamp that lkt open sourced? Might we want to have a cmdline option to set the date-time as an input, with a default for that time, and maybe an option that says, "now"?

E.g.

$ lkt build --modtime now ...
$ lkt build --modtime 20181229T175500Z ...

Or -timestamp or some other flag name, but that idea? And if not specified, just uses the date/time lkt was open-sourced?

[rn]

Yeah, was considering making the timestamp configurable. But rather than via the commandline, we should probably support this proposal and make it configurable via the environment variable SOURCE_DATE_EPOCH. Just want to get the basics in place and then iterate over refinements...

[deitch]

If that’s where it’s going as a standard, I guess. I like command line options as they are explicit. But not going to be a stickler about it. :-)

[justincormack]

LGTM as a first pass. I think setting all times to the git commit time of the linuxkit build itself might also be a reasonable option in future, but a static time is fine for now.

[rn]

I like the idea of using the git commit time as the default. That way it is different for different linuxkit versions. Might look at doing this as an improvement later along with support for SOURCE_DATE_EPOCH.