Ensure image SHA stays consistent when layer contents haven't changed

[tinkerborg]

Fixes #900 .

Description

omit uname/gname in tar headers

When using cache, the rootfs may not have been extracted. This prevents uname/gname from resolving as there is no /etc/password or /etc/group. This makes this layer unnecessarily differ from a cached layer which does contain this information. Omitting these should be consistent with Docker's behavior.

sort filesToAdd in TakeSnapshot

filesToAdd is sorted in TakeSnapshotFS, but not here. This makes ordering unpredictable within the layer's tarball, causing the SHA to differ even if layer contents haven't changed

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[tejal29]

Fixed build issue.
Thanks @tinkerborg I manually verified your PR and it works great!

[tejal29]

Build the same Dockerfile twice with --reproducible flag.

tejaldesai@@test$ container-diff diff image1.tar image.tar 

-----Size-----

Image size difference between image1.tar and image.tar: None
tejaldesai@@test$