Allow multiple destinations

[mschrupp]

Hello everyone,

I had the requirement to push an image to multiple container registries.
Since the image is already built it would make sense to push it to more than one registry (instead of building the image again with a different destination).

I recycled and existing type for multiple args and added a simple for-loop.
The missing os.Exit inside the loop made sense to me (why stop when only one of multiple pushes fail)
Tested this in my fork.

You can now specify multiple --destination flags.
If the tarpath is set, multiple tar-files (one for each destination) will be generated.

Please note that I didn't touch the README.md in this pull request.

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[mschrupp]

I signed it!

[priyawadhwa]

Thanks for contributing, I left a couple comments!

[priyawadhwa]

Could we instead pass the destinations into DoPush, and have the for loop there?

[priyawadhwa]

We probably want to add the os.Exit(1) here!

[googlebot]

CLAs look good, thanks!

[mschrupp]

Tested this, should be fine

[priyawadhwa]

Great, thanks!