Use standard archive/tar instead of docker's untar implementation

[ulyssessouza]

!!! Note that this PR is implemented based in another !!!
The only 2 exclusive files on this PR are:

• internal/packager/packing_test.go
• internal/tar/untar.go

- What I did
Substitute docker's untar package with a standard golang implementation

- How I did it
Adapted an implementation of untar to fit our needs and use it in TestPackInvocationImageContext

- How to verify it
Tests on packing_test.go passing without the need of running inside a container (just with make test and not make -f docker.Makefile test)

- Description for the changelog
Substitute docker's untar package with a standard golang implementation

- A picture of a cute animal

[vdemeester]

Question : Why is it required ?

• What are the reasons behind moving away form github.com/docker/docker/pkg/archive ?
• If it's related to lchown, I'm pretty sure using TarOptions field NoLchown should work.
• If not, I still believe it's better to fix/contribute upstream (it's even in the same docker org…) that duplicate this code.

[ulyssessouza]

@vdemeester The long story is that:

What are the reasons behind moving away form github.com/docker/docker/pkg/archive ?

The idea is to be able to run this test with just a make test and not make -f docker.Makefile test the first runs in the host and the second will be executed inside a container.
The problem of using github.com/docker/docker/pkg/archive here is that it assumes it's inside a container. Like when assuming that infos about UID and GID are located in /etc/subuid and /etc/subgid (as in idtools.go:L33-34)

If it's related to lchown, I'm pretty sure using TarOptions field NoLchown should work.

That was my first guess. The problem here is that according to idtools.go:L55 it not chown the exisiting files, but will systematically do it for the new ones.
The TarOptions::NoLchown is just used in archive.go:L651 that will prevent the os.Lchown in archive.go:L655 to be executed, but not the firstly described.

If not, I still believe it's better to fix/contribute upstream (it's even in the same docker org…) that duplicate this code.

My concern with this is that from what it looks, github.com/docker/docker/pkg/archive was built to be used in a different environment, which seems to be inside a container where it can read /etc/sub{uid|gid} to retrieve the user and group. Without this assumption, it should retrieve the username and group for different OSs, and also docker-app is compiled without CGO, so even a user.Current() work. So all of this makes me think its not buggy, but just thought to be used in a different context.

Given that, one possibility of contribution for the upstream would be add a new func in idtools.go which is similar to MkdirAllAndChownNew (given that it's a public func I cannot just change it's signature) that tells mkdirAs (having as side-effect the change of it's signature) to not chown even the new files. That would work.

Talking with @silvin-lubecki, he came with a simplier solution, that consists in just iterate through the entries in the archive, instead of really extracting and iterating through it.
This solution seems to be cleaner for our needs.

[thaJeztah]

We should probably be careful replacing this package; there may be special cases for backward compatibility in this package @tonistiigi @dmcgowan

[ulyssessouza]

@vdemeester And what about now?
I just pushed a simple solution described in the last paragraph of last message.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "std-untar" git@github.com:ulyssessouza/app.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842357813216
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (cnab-preview@133a2c6). Click here to learn what that means.
The diff coverage is 48.42%.

@@               Coverage Diff               @@
##             cnab-preview     #435   +/-   ##
===============================================
  Coverage                ?   59.04%           
===============================================
  Files                   ?       56           
  Lines                   ?     2803           
  Branches                ?        0           
===============================================
  Hits                    ?     1655           
  Misses                  ?      931           
  Partials                ?      217

Impacted Files	Coverage Δ
cmd/docker-app/init.go	100% <ø> (ø)
internal/names.go	100% <ø> (ø)
types/metadata/metadata.go	100% <ø> (ø)
types/parameters/opts.go	100% <ø> (ø)
cmd/docker-app/image-add.go	0% <0%> (ø)
cmd/docker-app/cnab.go	0% <0%> (ø)
cmd/docker-app/validate.go	88.88% <100%> (ø)
internal/renderer/renderer.go	100% <100%> (ø)
internal/packager/parameter.go	100% <100%> (ø)
loader/loader.go	79.24% <100%> (ø)
... and 21 more

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[ulyssessouza]

Closing it since it has been merged by #443