Support dockerfile agent declaration with filename

[zgyorffi]

I came across a limitation, the library does not support a jenkinsfile I am using even though it is completely valid and works fine on Jenkins. I've created this pull request to address this limitation so that I could continue the journey with this library. I am basically a newbie to groovy (just implemented a few jenkinsfiles in it so far) so open to any suggestions. I followed the pattern used for KubernetesAgentDeclaration and moved DockerfileAgentDeclaration in a dedicated class.

An example jenkinsfile which should be supported:

pipeline {
    stages {
        stage('Build') {
            agent {
                dockerfile {
                    filename 'Dockerfile'
                }
            }
        }
    }
}

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[nre-ableton]

Looks great, thanks!