Document the accepted inputs when configuring tags on spring-boot:build-image and bootBuildImage

[VitaliyKulikov]

• gradle plugin version: id("org.springframework.boot") version "2.7.5"
• dsl: kotlin
• snippet:

tasks.bootBuildImage {
  imageName = "sample"
  tags = listOf("tag1", "tag2")
}

• command: gradle bootBuildImage
• actual:

Successfully built image 'docker.io/library/sample:latest'
Successfully created image tag 'docker.io/library/tag1'
Successfully created image tag 'docker.io/library/tag2'

• expected:

Successfully built image 'docker.io/library/sample:latest'
Successfully created image tag 'docker.io/library/sample:tag1'
Successfully created image tag 'docker.io/library/sample:tag2'

[wilkinsona]

We know from #33002 that we have some users who expect the current behaviour. They have configured their tags like this:

bootBuildImage {
	imageName = "eu.gcr.io/company/imagename"
	builder = "paketobuildpacks/builder:tiny"
	if (System.getenv("IMAGE_TAGS")) {
		tags = (System.getenv("IMAGE_TAGS").split(",") as List<String>).collect { "$imageName:$it" }
	}
}

If we change it to work as shown in the expected output above, this configuration snippet would no longer work.

We're also aligned with the pack CLI which describes its --tag option thus:

  -t, --tag strings                 Additional tags to push the output image to.
                                    Tags should be in the format 'image:tag' or 'repository/image:tag'.
                                    Repeat for each tag in order, or supply once by comma-separated list

In our implementation, each tag becomes an ImageReference which means that things are a little more flexible due to its implementation:

Create a new ImageReference from the given value. The following value forms can be used:

• name (maps to docker.io/library/name)
• domain/name
• domain:port/name
• domain:port/name:tag
• domain:port/name@digest

I think we should update the Maven and Gradle plugin documentation to describe the behavior of tags in more detail.