If no version is specified, the tag should default to "latest" instead of "unspecified"

Description of the issue:

When running ./gradlew jibDockerBuild the Docke rimage is always also tagged with “unspecified” even if a tag is specified via jib.to.tags.

Expected behavior:

Only the tags specified via jib.to.tags are used to tag the Docker image.

Steps to reproduce:

Run ./gradlew jibDockerBuild with the configuration as mentioned below. See https://github.com/heremaps/oss-review-toolkit/blob/jib/cli/build.gradle to reproduce.

Environment:

Ubuntu 18.04

jib-gradle-plugin Configuration:

plugins {
    id 'com.google.cloud.tools.jib' version '0.9.11'
}

jib.to.tags = ['latest']

Log output:

Parallel execution with configuration on demand is an incubating feature.
Tagging image with generated image reference cli:unspecified. If you'd like to specify a different tag, you can set the jib.to.image parameter in your build.gradle, or use the --image=<MY IMAGE> commandline flag.
warning: Base image 'gcr.io/distroless/java' does not use a specific image digest - build may not be reproducible

Containerizing application to Docker daemon as cli:unspecified, cli...

Getting base image gcr.io/distroless/java...
Building dependencies layer...
Building classes layer...
Finalizing...
Loading to Docker daemon...

Container entrypoint set to [java, -cp, /app/resources:/app/classes:/app/libs/*, com.here.ort.Main]

Built image to Docker daemon as cli:unspecified, cli

BUILD SUCCESSFUL in 9s
68 actionable tasks: 1 executed, 67 up-to-date

[chanseokoh]

Makes sense, especially when considering that jib.to.image = 'cli' gives you cli:latest instead of cli:unspecified.

[coollog]

Yea, I think we can change the behavior to not include a default tag if the version is unspecified.