add possibility to give multiple registry mirrors via environment variable (KANIKO_REGISTRY_MIRROR)

[tldev-de]

Actual behavior
The flag --registry-mirror can be used multiple times to give multiple registry mirrors into the build process. Sadly, the corresponding environment variable KANIKO_REGISTRY_MIRROR accepts only one registry mirror. When I try to set multiple values via any separator, I get the following error:

WARN[0000] Failed to retrieve image alpine/curl from remapped registry docker-mirror01.extern.XXX dockerhub.intranet.XXX docker-mirror02.extern.XXX: unable to complete operation after 0 attempts, last error: parse "https://docker-mirror01.extern.XXX dockerhub.intranet.XXX docker-mirror02.extern.XXX/v2/": invalid character " " in host name. Will try with the next registry, or fallback to the original registry.

Obviously, the environment variable does not support multiple registry mirrors - see root.go:

kaniko/cmd/executor/cmd/root.go

Line 72 in 45a6d2d

if val, ok := os.LookupEnv("KANIKO_REGISTRY_MIRROR"); ok {

Expected behavior
It should be possible to add multiple registry mirrors by a separator, e.g. any char, that is not used in urls. In my opinion, a space would be a good choice.

Since this looks like a small change and I'm getting familiar with golang, I would love to implement this feature.

To Reproduce
Steps to reproduce the behavior:

1. download the provided dockerfile
2. run kaniko with multiple registry mirrors, e.g.:

docker run -it --rm -e "KANIKO_REGISTRY_MIRROR=docker-mirror01.extern.XXX dockerhub.intranet.XXX docker-mirror02.extern.XXX" -v `pwd`:/workspace gcr.io/kaniko-project/executor:latest --dockerfile=Dockerfile --destination=image --tarPath=/workspace/image.tar --no-push

Additional Information

• Dockerfile

FROM alpine/curl
RUN curl https://github.com/docker-library/hello-world/raw/master/amd64/hello-world/hello -o /hello
CMD ["/hello"]

• Build Context
  nothing needed
• Kaniko Image (fully qualified with digest)
  any (gcr.io/kaniko-project/executor:latest)

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

[babs]

Take a look at registry-map witch allows exactly that

See https://github.com/GoogleContainerTools/kaniko#flag---registry-map
index.docker.io=docker-io.mirrors.corp.net;index.docker.io=mirror.gcr.io;gcr.io=127.0.0.1 will try docker-io.mirrors.corp.net then mirror.gcr.io for index.docker.io and 127.0.0.1 for gcr.io