Add multiarch support

[slamd64]

ARM64 architecture becomes very popular and AWS EKS/ECR already supports this. So, syncing AMD64+ARM64 is good idea.
https://docs.docker.com/docker-for-mac/multi-arch/

Skopeo is already supports this feature. So, it needs to bypass "--all" from Dregsy command line to Skopeo.

skopeo copy
options:
--all
If source-image refers to a list of images, instead of copying just the image which matches the current OS and architecture (subject to the use of the global --override-os, --override-arch and --override-variant options), attempt to copy all of the images in the list, and the list itself.

[slamd64]

Hi!

Do you have any plans for multiarch? :)

[xelalexv]

I think this would be a really useful enhancement. Problem is, I really need to get the filtering stuff out first. Unfortunately I don't have that much time to work on dregsy, so I can't promise when this is going to happen.

[xelalexv]

I finally got around to working on this. When syncing from a multi-platform source image, you will have two options now - select one platform image to sync, or sync all available platform images. This can be defined per mapping, for example:

  mappings:
  - from: library/busybox
    to: library/busybox-arm64
    tags: ['latest']
    platform: linux/arm64/v8

This syncs the linux/arm64/v8 from a busybox multi-platform image. To select all platforms, specify platform: all. all however only works with the Skopeo relay. If you want to sync a selection of platform images from the same multi-platform source image, several mappings with according platform settings can be defined. You need to be careful however not to map them into the same destination, i.e. use different to settings. Otherwise, the synced platform images will "overwrite" each other, with only the last image synced being available from the target repository. For example:

  mappings:
  - from: library/busybox
    to: library/busybox-arm64
    tags: ['latest']
    platform: linux/arm64/v8
  - from: library/busybox
    to: library/busybox-amd64
    tags: ['latest']
    platform: linux/amd64

[xelalexv]

Implemented with PR #43.