Expose default value for imageName in bootBuildImage task

[scottfrederick]

When combining the Spring Boot Gradle plugin's bootBuildImage task with other Docker-related plugin tasks, it is convenient to be able to reference the default bootBuildImage property imageName. For example:

plugins {
	id 'org.springframework.boot' version '...'
	id 'com.bmuschko.docker-remote-api' version '7.1.0'
	...
}

tasks.register('tagImage', DockerTagImage) {
	dependsOn 'bootBuildImage'
	imageId = bootBuildImage.imageName
	repository = "example.com/${project.name}"
	tag = "latest"
}

Currently, bootBuildImage.imageName returns null if not explicitly set in the build configuration, which results in an error when the tagImage task is run:

A problem was found with the configuration of task ':tagImage' (type 'DockerTagImage').
  - Type 'DockerTagImage' property 'imageId' doesn't have a configured value.

    Reason: This property isn't marked as optional and no value has been configured.