Add flag to remap registries for any registry mirror (#2935)

* Add flag to remap registries for any registry mirror

The purpose of this PR is to add an option to remap registries, a kind of generalized `--registry-mirror`.
This is helpful for air-gapped environments and/or when local registry mirrors are available (not limited to docker.io).
This allows user to reference any images without having to change their location.
It also permit to separate infra related configuration (the mirrors) from CI/CD pipeline definition by using an environment variable for example (the reason behind the early return if flag provided but empty).
Therefore you can have a pipeline calling kaniko with `--registry-map=$REGISTRY_MAP` and have the `REGISTRY_MAP` populated via the runner's env by another team, and the absence of env wouldn't trigger a failure, it makes the pipeline env independent.

I've also considered the option of environment variables directly but it doesn't seems to be in kaniko's philosophy.

This makes quite some duplicated code :/ One option to keep the mirror flag and behavior would be to use only one codebase and convert `--registry-mirror=VALUE` to `--registry-map=index.docker.io=VALUE` internally. Suggestions welcome!

* Configure logging config sooner to be able to use it in flag parsing

* Replace registry mirrors by maps logic and use env var

* Add env vars to README.md

* Fix test

cmd/executor/cmd/root.go

@@ -34,6 +34,7 @@ import (
 	"github.com/GoogleContainerTools/kaniko/pkg/util"
 	"github.com/GoogleContainerTools/kaniko/pkg/util/proc"
 	"github.com/containerd/containerd/platforms"
+	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -70,6 +71,21 @@ func validateFlags() {
 		opts.RegistryMirrors.Set(val)
 	}
 
+	// Allow setting --registry-maps using an environment variable.
+	if val, ok := os.LookupEnv("KANIKO_REGISTRY_MAP"); ok {
+		opts.RegistryMaps.Set(val)
+	}
+
+	for _, target := range opts.RegistryMirrors {
+		opts.RegistryMaps.Set(fmt.Sprintf("%s=%s", name.DefaultRegistry, target))
+	}
+
+	if len(opts.RegistryMaps) > 0 {
+		for src, dsts := range opts.RegistryMaps {
+			logrus.Debugf("registry-map remaps %s to %s.", src, strings.Join(dsts, ", "))
+		}
+	}
+
 	// Default the custom platform flag to our current platform, and validate it.
 	if opts.CustomPlatform == "" {
 		opts.CustomPlatform = platforms.Format(platforms.Normalize(platforms.DefaultSpec()))
@@ -85,6 +101,10 @@ var RootCmd = &cobra.Command{
 	PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 		if cmd.Use == "executor" {
 
+			if err := logging.Configure(logLevel, logFormat, logTimestamp); err != nil {
+				return err
+			}
+
 			validateFlags()
 
 			// Command line flag takes precedence over the KANIKO_DIR environment variable.
@@ -99,10 +119,6 @@ var RootCmd = &cobra.Command{
 
 			resolveEnvironmentBuildArgs(opts.BuildArgs, os.Getenv)
 
-			if err := logging.Configure(logLevel, logFormat, logTimestamp); err != nil {
-				return err
-			}
-
 			if !opts.NoPush && len(opts.Destinations) == 0 {
 				return errors.New("you must provide --destination, or use --no-push")
 			}
@@ -238,6 +254,8 @@ func addKanikoOptionsFlags() {
 	RootCmd.PersistentFlags().VarP(&opts.RegistriesCertificates, "registry-certificate", "", "Use the provided certificate for TLS communication with the given registry. Expected format is 'my.registry.url=/path/to/the/server/certificate'.")
 	opts.RegistriesClientCertificates = make(map[string]string)
 	RootCmd.PersistentFlags().VarP(&opts.RegistriesClientCertificates, "registry-client-cert", "", "Use the provided client certificate for mutual TLS (mTLS) communication with the given registry. Expected format is 'my.registry.url=/path/to/client/cert,/path/to/client/key'.")
+	opts.RegistryMaps = make(map[string][]string)
+	RootCmd.PersistentFlags().VarP(&opts.RegistryMaps, "registry-map", "", "Registry map of mirror to use as pull-through cache instead. Expected format is 'orignal.registry=new.registry;other-original.registry=other-remap.registry'")
 	RootCmd.PersistentFlags().VarP(&opts.RegistryMirrors, "registry-mirror", "", "Registry mirror to use as pull-through cache instead of docker.io. Set it repeatedly for multiple mirrors.")
 	RootCmd.PersistentFlags().BoolVarP(&opts.SkipDefaultRegistryFallback, "skip-default-registry-fallback", "", false, "If an image is not found on any mirrors (defined with registry-mirror) do not fallback to the default registry. If registry-mirror is not defined, this flag is ignored.")
 	RootCmd.PersistentFlags().BoolVarP(&opts.IgnoreVarRun, "ignore-var-run", "", true, "Ignore /var/run directory when taking image snapshot. Set it to false to preserve /var/run/ in destination image.")