fix [JENKINS-71060] Error: Tag length > 128

[Robbos]

Ticket: jenkinsci/docker-workflow-plugin#706

This PR ensures we don't take the colon as part of the docker tag.
It lead to this error if the tag name was 128 chars long:

	at hudson.util.FormValidation._errorWithMarkup(FormValidation.java:274)
	at hudson.util.FormValidation.errorWithMarkup(FormValidation.java:260)
	at hudson.util.FormValidation.error(FormValidation.java:150)
	at PluginClassLoader for docker-commons//org.jenkinsci.plugins.docker.commons.credentials.ImageNameValidator.validateTag(ImageNameValidator.java:240)
	at PluginClassLoader for docker-commons//org.jenkinsci.plugins.docker.commons.credentials.ImageNameValidator.validateUserAndRepo(ImageNameValidator.java:117)
	at PluginClassLoader for docker-commons//org.jenkinsci.plugins.docker.commons.credentials.ImageNameValidator.checkUserAndRepo(ImageNameValidator.java:142)

Testing done

New unit test was added: testSplitUserAndRepo referring to the ticket JENKINS-71060.

I ran the project locally.
On master with this simple pipeline, it was failing with the error from JENKINS-71060,
On my branch it is not.
Feel free to double check with a simple pipeline like this

pipeline {
    agent any
    stages {
        stage('Docker test') {
            steps {
                script {
                    docker.build('dockerdev.myorg.com/dockerdev-local/myproduct0/demo-pipeline-hello-world:demo-pipeline-hello-world-github-myorgmyproduct0-org-demo-pipeline-test-2fab1234-56789_feat_ci_-a1b2c3d4e5f6789012345678abcdef01')
                }
            }
        }
    }
}

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[jglick]

docker-workflow is up for adoption, so PRs are unlikely to be looked at unless there is some critical regression (e.g., introduced by a containerd or Jenkins update) or something needed for the broader Jenkins ecosystem (e.g., Commons Lang 2 dep). This lib functionality applies only to that plugin so far as I know.