Fixes#1473
The initial implementation of the registry mirror only allowed a single mirror, and if pulling from the mirror failed, the build would fail.
This change introduces:
- multiple registry mirrors instead of a single one
- fallback if an image can't be pulled from a registry
This is the same behavior as the docker daemon and will allow using a registry mirror such as `mirror.gcr.io` which is incomplete and doesn't have all the content that the default registry on docker.io has.
Note that there are no changes in the CLI flags, the `--registry-mirror` flag is still valid. But now it can be used multiple times to set up more than one registry mirror.
Co-authored-by: Tejal Desai <tejaldesai@google.com>
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