Add support for DockerImage/alwaysPull

[dblock]

Feature Request

Add support for alwaysPull.

https://github.com/jenkinsci/docker-workflow-plugin/blob/b174d46226ef1095903f2e789355a3b216b46dda/src/main/java/org/jenkinsci/plugins/docker/workflow/declarative/DockerPipeline.java

            agent {
                docker {
                    label ...
                    image ...
                    alwaysPull true/false
                }
            }