Support creation of Docker tags

Description of the issue: The Jib Gradle/Maven plugins should support the creation of Docker tags without building a new image. This is important for promotion workflows where an image gains new tags while being promoted through the stages.

Expected behavior:

Something like the command below creates a tag in the repository repo:

gradle jibTag --source "repo/img:$GIT_COMMIT" --target "repo/img:$GIT_TAG"

[coollog]

Hi @gluehbirnenkopf , this seems like a use case that is outside the scope of Jib, since Jib is intended to be just a container builder for Java.

For a standalone tool to copy remote images, I would recommend crane.

If you’d like to copy an image as part of your Gradle build, you could use Jib Core:

// Imports Jib Core as a library to use in this build script.
buildscript {
  repositories {
    mavenCentral()
  }
  dependencies {
    classpath 'com.google.cloud.tools:jib-core:0.1.1'
  }
}

import com.google.cloud.tools.jib.api.Jib
import com.google.cloud.tools.jib.api.Containerizer
import com.google.cloud.tools.jib.api.RegistryImage
import com.google.cloud.tools.jib.frontend.CredentialRetrieverFactory
import com.google.cloud.tools.jib.image.ImageReference

// 'jibcopy' copies a container image from `sourceImage` to `targetImage`.
task('jibcopy') {
  doLast { 
    def sourceImage = '<source image reference>'
    def targetImage = '<target image reference>'
    Jib
        // Starts with the existing image.
        .from(sourceImage)
        // Performs the containerization.
        .containerize(
            Containerizer.to(
                // Tells Jib to containerize to targetImage's registry. 
                RegistryImage
                    .named(targetImage)
                    // Tells Jib to get registry credentials from a Docker config.
                    .addCredentialRetriever(
                        CredentialRetrieverFactory
                            .forImage(ImageReference.parse(targetImage))
                            .dockerConfig())))
  }
}

[coollog]

Hi @az82 , in Jib, you can actually push the same image with a different tag (just change the to.image field, or create profiles to use multiple tags). Even though it might seem like Jib is building a new image - it skips all the actual building and just creates a new manifest for the new tag. This is a benefit we get from the reproducibility aspect of Jib, where the image layers that were pushed in the first tag would get reused by the new tags.